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bpf: verifier (add docs)
this patch adds all of eBPF verfier documentation and empty bpf_check() The end goal for the verifier is to statically check safety of the program. Verifier will catch: - loops - out of range jumps - unreachable instructions - invalid instructions - uninitialized register access - uninitialized stack access - misaligned stack access - out of range stack access - invalid calling convention More details in Documentation/networking/filter.txt Signed-off-by: Alexei Starovoitov <ast@plumgrid.com> Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Alexei Starovoitov
David S. Miller
parent
0a542a86d7
commit
51580e798c
@ -1001,6 +1001,99 @@ instruction that loads 64-bit immediate value into a dst_reg.
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Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM which loads
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32-bit immediate value into a register.
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eBPF verifier
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-------------
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The safety of the eBPF program is determined in two steps.
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First step does DAG check to disallow loops and other CFG validation.
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In particular it will detect programs that have unreachable instructions.
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(though classic BPF checker allows them)
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Second step starts from the first insn and descends all possible paths.
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It simulates execution of every insn and observes the state change of
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registers and stack.
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At the start of the program the register R1 contains a pointer to context
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and has type PTR_TO_CTX.
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If verifier sees an insn that does R2=R1, then R2 has now type
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PTR_TO_CTX as well and can be used on the right hand side of expression.
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If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=UNKNOWN_VALUE,
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since addition of two valid pointers makes invalid pointer.
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(In 'secure' mode verifier will reject any type of pointer arithmetic to make
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sure that kernel addresses don't leak to unprivileged users)
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If register was never written to, it's not readable:
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bpf_mov R0 = R2
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bpf_exit
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will be rejected, since R2 is unreadable at the start of the program.
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After kernel function call, R1-R5 are reset to unreadable and
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R0 has a return type of the function.
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Since R6-R9 are callee saved, their state is preserved across the call.
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bpf_mov R6 = 1
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bpf_call foo
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bpf_mov R0 = R6
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bpf_exit
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is a correct program. If there was R1 instead of R6, it would have
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been rejected.
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load/store instructions are allowed only with registers of valid types, which
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are PTR_TO_CTX, PTR_TO_MAP, FRAME_PTR. They are bounds and alignment checked.
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For example:
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bpf_mov R1 = 1
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bpf_mov R2 = 2
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bpf_xadd *(u32 *)(R1 + 3) += R2
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bpf_exit
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will be rejected, since R1 doesn't have a valid pointer type at the time of
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execution of instruction bpf_xadd.
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At the start R1 type is PTR_TO_CTX (a pointer to generic 'struct bpf_context')
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A callback is used to customize verifier to restrict eBPF program access to only
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certain fields within ctx structure with specified size and alignment.
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For example, the following insn:
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bpf_ld R0 = *(u32 *)(R6 + 8)
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intends to load a word from address R6 + 8 and store it into R0
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If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know
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that offset 8 of size 4 bytes can be accessed for reading, otherwise
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the verifier will reject the program.
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If R6=FRAME_PTR, then access should be aligned and be within
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stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8,
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so it will fail verification, since it's out of bounds.
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The verifier will allow eBPF program to read data from stack only after
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it wrote into it.
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Classic BPF verifier does similar check with M[0-15] memory slots.
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For example:
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bpf_ld R0 = *(u32 *)(R10 - 4)
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bpf_exit
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is invalid program.
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Though R10 is correct read-only register and has type FRAME_PTR
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and R10 - 4 is within stack bounds, there were no stores into that location.
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Pointer register spill/fill is tracked as well, since four (R6-R9)
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callee saved registers may not be enough for some programs.
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Allowed function calls are customized with bpf_verifier_ops->get_func_proto()
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The eBPF verifier will check that registers match argument constraints.
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After the call register R0 will be set to return type of the function.
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Function calls is a main mechanism to extend functionality of eBPF programs.
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Socket filters may let programs to call one set of functions, whereas tracing
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filters may allow completely different set.
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If a function made accessible to eBPF program, it needs to be thought through
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from safety point of view. The verifier will guarantee that the function is
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called with valid arguments.
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seccomp vs socket filters have different security restrictions for classic BPF.
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Seccomp solves this by two stage verifier: classic BPF verifier is followed
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by seccomp verifier. In case of eBPF one configurable verifier is shared for
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all use cases.
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See details of eBPF verifier in kernel/bpf/verifier.c
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eBPF maps
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---------
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'maps' is a generic storage of different types for sharing data between kernel
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@ -1040,6 +1133,137 @@ The map is defined by:
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. key size in bytes
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. value size in bytes
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Understanding eBPF verifier messages
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------------------------------------
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The following are few examples of invalid eBPF programs and verifier error
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messages as seen in the log:
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Program with unreachable instructions:
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static struct bpf_insn prog[] = {
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BPF_EXIT_INSN(),
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BPF_EXIT_INSN(),
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};
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Error:
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unreachable insn 1
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Program that reads uninitialized register:
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BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
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BPF_EXIT_INSN(),
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Error:
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0: (bf) r0 = r2
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R2 !read_ok
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Program that doesn't initialize R0 before exiting:
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BPF_MOV64_REG(BPF_REG_2, BPF_REG_1),
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BPF_EXIT_INSN(),
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Error:
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0: (bf) r2 = r1
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1: (95) exit
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R0 !read_ok
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Program that accesses stack out of bounds:
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BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0),
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BPF_EXIT_INSN(),
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Error:
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0: (7a) *(u64 *)(r10 +8) = 0
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invalid stack off=8 size=8
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Program that doesn't initialize stack before passing its address into function:
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BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
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BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
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BPF_LD_MAP_FD(BPF_REG_1, 0),
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BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
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BPF_EXIT_INSN(),
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Error:
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0: (bf) r2 = r10
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1: (07) r2 += -8
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2: (b7) r1 = 0x0
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3: (85) call 1
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invalid indirect read from stack off -8+0 size 8
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Program that uses invalid map_fd=0 while calling to map_lookup_elem() function:
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BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
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BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
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BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
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BPF_LD_MAP_FD(BPF_REG_1, 0),
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BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
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BPF_EXIT_INSN(),
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Error:
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0: (7a) *(u64 *)(r10 -8) = 0
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1: (bf) r2 = r10
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2: (07) r2 += -8
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3: (b7) r1 = 0x0
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4: (85) call 1
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fd 0 is not pointing to valid bpf_map
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Program that doesn't check return value of map_lookup_elem() before accessing
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map element:
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BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
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BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
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BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
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BPF_LD_MAP_FD(BPF_REG_1, 0),
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BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
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BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
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BPF_EXIT_INSN(),
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Error:
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0: (7a) *(u64 *)(r10 -8) = 0
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1: (bf) r2 = r10
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2: (07) r2 += -8
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3: (b7) r1 = 0x0
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4: (85) call 1
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5: (7a) *(u64 *)(r0 +0) = 0
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R0 invalid mem access 'map_value_or_null'
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Program that correctly checks map_lookup_elem() returned value for NULL, but
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accesses the memory with incorrect alignment:
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BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
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BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
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BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
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BPF_LD_MAP_FD(BPF_REG_1, 0),
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BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
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BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
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BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0),
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BPF_EXIT_INSN(),
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Error:
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0: (7a) *(u64 *)(r10 -8) = 0
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1: (bf) r2 = r10
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2: (07) r2 += -8
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3: (b7) r1 = 1
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4: (85) call 1
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5: (15) if r0 == 0x0 goto pc+1
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R0=map_ptr R10=fp
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6: (7a) *(u64 *)(r0 +4) = 0
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misaligned access off 4 size 8
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Program that correctly checks map_lookup_elem() returned value for NULL and
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accesses memory with correct alignment in one side of 'if' branch, but fails
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to do so in the other side of 'if' branch:
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BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
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BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
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BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
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BPF_LD_MAP_FD(BPF_REG_1, 0),
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BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
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BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
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BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
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BPF_EXIT_INSN(),
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BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1),
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BPF_EXIT_INSN(),
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Error:
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0: (7a) *(u64 *)(r10 -8) = 0
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1: (bf) r2 = r10
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2: (07) r2 += -8
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3: (b7) r1 = 1
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4: (85) call 1
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5: (15) if r0 == 0x0 goto pc+2
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R0=map_ptr R10=fp
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6: (7a) *(u64 *)(r0 +0) = 0
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7: (95) exit
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from 5 to 8: R0=imm0 R10=fp
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8: (7a) *(u64 *)(r0 +0) = 1
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R0 invalid mem access 'imm'
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Testing
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-------
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@ -83,5 +83,7 @@ struct bpf_prog_aux {
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void bpf_prog_put(struct bpf_prog *prog);
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struct bpf_prog *bpf_prog_get(u32 ufd);
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/* verify correctness of eBPF program */
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int bpf_check(struct bpf_prog *fp, union bpf_attr *attr);
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#endif /* _LINUX_BPF_H */
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@ -1 +1 @@
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obj-y := core.o syscall.o
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obj-y := core.o syscall.o verifier.o
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@ -507,7 +507,7 @@ static int bpf_prog_load(union bpf_attr *attr)
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goto free_prog;
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/* run eBPF verifier */
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/* err = bpf_check(prog, tb); */
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err = bpf_check(prog, attr);
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if (err < 0)
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goto free_used_maps;
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133
kernel/bpf/verifier.c
Normal file
133
kernel/bpf/verifier.c
Normal file
@ -0,0 +1,133 @@
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/* Copyright (c) 2011-2014 PLUMgrid, http://plumgrid.com
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of version 2 of the GNU General Public
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* License as published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*/
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#include <linux/kernel.h>
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#include <linux/types.h>
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#include <linux/slab.h>
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#include <linux/bpf.h>
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#include <linux/filter.h>
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#include <net/netlink.h>
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#include <linux/file.h>
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#include <linux/vmalloc.h>
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/* bpf_check() is a static code analyzer that walks eBPF program
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* instruction by instruction and updates register/stack state.
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* All paths of conditional branches are analyzed until 'bpf_exit' insn.
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*
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* The first pass is depth-first-search to check that the program is a DAG.
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* It rejects the following programs:
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* - larger than BPF_MAXINSNS insns
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* - if loop is present (detected via back-edge)
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* - unreachable insns exist (shouldn't be a forest. program = one function)
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* - out of bounds or malformed jumps
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* The second pass is all possible path descent from the 1st insn.
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* Since it's analyzing all pathes through the program, the length of the
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* analysis is limited to 32k insn, which may be hit even if total number of
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* insn is less then 4K, but there are too many branches that change stack/regs.
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* Number of 'branches to be analyzed' is limited to 1k
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*
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* On entry to each instruction, each register has a type, and the instruction
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* changes the types of the registers depending on instruction semantics.
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* If instruction is BPF_MOV64_REG(BPF_REG_1, BPF_REG_5), then type of R5 is
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* copied to R1.
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*
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* All registers are 64-bit.
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* R0 - return register
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* R1-R5 argument passing registers
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* R6-R9 callee saved registers
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* R10 - frame pointer read-only
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*
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* At the start of BPF program the register R1 contains a pointer to bpf_context
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* and has type PTR_TO_CTX.
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*
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* Verifier tracks arithmetic operations on pointers in case:
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* BPF_MOV64_REG(BPF_REG_1, BPF_REG_10),
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* BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, -20),
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* 1st insn copies R10 (which has FRAME_PTR) type into R1
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* and 2nd arithmetic instruction is pattern matched to recognize
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* that it wants to construct a pointer to some element within stack.
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* So after 2nd insn, the register R1 has type PTR_TO_STACK
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* (and -20 constant is saved for further stack bounds checking).
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* Meaning that this reg is a pointer to stack plus known immediate constant.
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*
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* Most of the time the registers have UNKNOWN_VALUE type, which
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* means the register has some value, but it's not a valid pointer.
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* (like pointer plus pointer becomes UNKNOWN_VALUE type)
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*
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* When verifier sees load or store instructions the type of base register
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* can be: PTR_TO_MAP_VALUE, PTR_TO_CTX, FRAME_PTR. These are three pointer
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* types recognized by check_mem_access() function.
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*
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* PTR_TO_MAP_VALUE means that this register is pointing to 'map element value'
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* and the range of [ptr, ptr + map's value_size) is accessible.
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*
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* registers used to pass values to function calls are checked against
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* function argument constraints.
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*
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* ARG_PTR_TO_MAP_KEY is one of such argument constraints.
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* It means that the register type passed to this function must be
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* PTR_TO_STACK and it will be used inside the function as
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* 'pointer to map element key'
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*
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* For example the argument constraints for bpf_map_lookup_elem():
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* .ret_type = RET_PTR_TO_MAP_VALUE_OR_NULL,
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* .arg1_type = ARG_CONST_MAP_PTR,
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* .arg2_type = ARG_PTR_TO_MAP_KEY,
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*
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* ret_type says that this function returns 'pointer to map elem value or null'
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* function expects 1st argument to be a const pointer to 'struct bpf_map' and
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* 2nd argument should be a pointer to stack, which will be used inside
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* the helper function as a pointer to map element key.
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*
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* On the kernel side the helper function looks like:
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* u64 bpf_map_lookup_elem(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
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* {
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* struct bpf_map *map = (struct bpf_map *) (unsigned long) r1;
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* void *key = (void *) (unsigned long) r2;
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* void *value;
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*
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* here kernel can access 'key' and 'map' pointers safely, knowing that
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* [key, key + map->key_size) bytes are valid and were initialized on
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* the stack of eBPF program.
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* }
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*
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* Corresponding eBPF program may look like:
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* BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), // after this insn R2 type is FRAME_PTR
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* BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4), // after this insn R2 type is PTR_TO_STACK
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* BPF_LD_MAP_FD(BPF_REG_1, map_fd), // after this insn R1 type is CONST_PTR_TO_MAP
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* BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
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* here verifier looks at prototype of map_lookup_elem() and sees:
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* .arg1_type == ARG_CONST_MAP_PTR and R1->type == CONST_PTR_TO_MAP, which is ok,
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* Now verifier knows that this map has key of R1->map_ptr->key_size bytes
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*
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* Then .arg2_type == ARG_PTR_TO_MAP_KEY and R2->type == PTR_TO_STACK, ok so far,
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* Now verifier checks that [R2, R2 + map's key_size) are within stack limits
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* and were initialized prior to this call.
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* If it's ok, then verifier allows this BPF_CALL insn and looks at
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* .ret_type which is RET_PTR_TO_MAP_VALUE_OR_NULL, so it sets
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* R0->type = PTR_TO_MAP_VALUE_OR_NULL which means bpf_map_lookup_elem() function
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* returns ether pointer to map value or NULL.
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*
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* When type PTR_TO_MAP_VALUE_OR_NULL passes through 'if (reg != 0) goto +off'
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* insn, the register holding that pointer in the true branch changes state to
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* PTR_TO_MAP_VALUE and the same register changes state to CONST_IMM in the false
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||||
* branch. See check_cond_jmp_op().
|
||||
*
|
||||
* After the call R0 is set to return type of the function and registers R1-R5
|
||||
* are set to NOT_INIT to indicate that they are no longer readable.
|
||||
*/
|
||||
|
||||
int bpf_check(struct bpf_prog *prog, union bpf_attr *attr)
|
||||
{
|
||||
int ret = -EINVAL;
|
||||
|
||||
return ret;
|
||||
}
|
||||
Loading…
Reference in New Issue
Block a user