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Documentation: describe the new eBPF verifier value tracking behaviour

Browse Source 
Also bring the eBPF documentation up to date in other ways.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Edward Cree 2017-08-07 15:30:09 +01:00 committed by David S. Miller
parent 69c4e8ada6
commit 0cbf474165
1 changed files with 104 additions and 18 deletions
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122
Documentation/networking/filter.txt
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@ -793,7 +793,7 @@ Some core changes of the new internal format:
bpf_exit
After the call the registers R1-R5 contain junk values and cannot be read.
In the future an eBPF verifier can be used to validate internal BPF programs.
An in-kernel eBPF verifier is used to validate internal BPF programs.
Also in the new design, eBPF is limited to 4096 insns, which means that any
program will terminate quickly and will only call a fixed number of kernel
@ -1017,7 +1017,7 @@ At the start of the program the register R1 contains a pointer to context
and has type PTR_TO_CTX.
If verifier sees an insn that does R2=R1, then R2 has now type
PTR_TO_CTX as well and can be used on the right hand side of expression.
If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=UNKNOWN_VALUE,
If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=SCALAR_VALUE,
since addition of two valid pointers makes invalid pointer.
(In 'secure' mode verifier will reject any type of pointer arithmetic to make
sure that kernel addresses don't leak to unprivileged users)
@ -1039,7 +1039,7 @@ is a correct program. If there was R1 instead of R6, it would have
been rejected.
load/store instructions are allowed only with registers of valid types, which
are PTR_TO_CTX, PTR_TO_MAP, FRAME_PTR. They are bounds and alignment checked.
are PTR_TO_CTX, PTR_TO_MAP, PTR_TO_STACK. They are bounds and alignment checked.
For example:
bpf_mov R1 = 1
bpf_mov R2 = 2
@ -1058,7 +1058,7 @@ intends to load a word from address R6 + 8 and store it into R0
If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know
that offset 8 of size 4 bytes can be accessed for reading, otherwise
the verifier will reject the program.
If R6=FRAME_PTR, then access should be aligned and be within
If R6=PTR_TO_STACK, then access should be aligned and be within
stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8,
so it will fail verification, since it's out of bounds.
@ -1069,7 +1069,7 @@ For example:
bpf_ld R0 = *(u32 *)(R10 - 4)
bpf_exit
is invalid program.
Though R10 is correct read-only register and has type FRAME_PTR
Though R10 is correct read-only register and has type PTR_TO_STACK
and R10 - 4 is within stack bounds, there were no stores into that location.
Pointer register spill/fill is tracked as well, since four (R6-R9)
@ -1094,6 +1094,71 @@ all use cases.
See details of eBPF verifier in kernel/bpf/verifier.c
Register value tracking
-----------------------
In order to determine the safety of an eBPF program, the verifier must track
the range of possible values in each register and also in each stack slot.
This is done with 'struct bpf_reg_state', defined in include/linux/
bpf_verifier.h, which unifies tracking of scalar and pointer values. Each
register state has a type, which is either NOT_INIT (the register has not been
written to), SCALAR_VALUE (some value which is not usable as a pointer), or a
pointer type. The types of pointers describe their base, as follows:
PTR_TO_CTX Pointer to bpf_context.
CONST_PTR_TO_MAP Pointer to struct bpf_map. "Const" because arithmetic
on these pointers is forbidden.
PTR_TO_MAP_VALUE Pointer to the value stored in a map element.
PTR_TO_MAP_VALUE_OR_NULL
Either a pointer to a map value, or NULL; map accesses
(see section 'eBPF maps', below) return this type,
which becomes a PTR_TO_MAP_VALUE when checked != NULL.
Arithmetic on these pointers is forbidden.
PTR_TO_STACK Frame pointer.
PTR_TO_PACKET skb->data.
PTR_TO_PACKET_END skb->data + headlen; arithmetic forbidden.
However, a pointer may be offset from this base (as a result of pointer
arithmetic), and this is tracked in two parts: the 'fixed offset' and 'variable
offset'. The former is used when an exactly-known value (e.g. an immediate
operand) is added to a pointer, while the latter is used for values which are
not exactly known. The variable offset is also used in SCALAR_VALUEs, to track
the range of possible values in the register.
The verifier's knowledge about the variable offset consists of:
* minimum and maximum values as unsigned
* minimum and maximum values as signed
* knowledge of the values of individual bits, in the form of a 'tnum': a u64
'mask' and a u64 'value'. 1s in the mask represent bits whose value is unknown;
1s in the value represent bits known to be 1. Bits known to be 0 have 0 in both
mask and value; no bit should ever be 1 in both. For example, if a byte is read
into a register from memory, the register's top 56 bits are known zero, while
the low 8 are unknown - which is represented as the tnum (0x0; 0xff). If we
then OR this with 0x40, we get (0x40; 0xcf), then if we add 1 we get (0x0;
0x1ff), because of potential carries.
Besides arithmetic, the register state can also be updated by conditional
branches. For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch
it will have a umin_value (unsigned minimum value) of 9, whereas in the 'false'
branch it will have a umax_value of 8. A signed compare (with BPF_JSGT or
BPF_JSGE) would instead update the signed minimum/maximum values. Information
from the signed and unsigned bounds can be combined; for instance if a value is
first tested < 8 and then tested s> 4, the verifier will conclude that the value
is also > 4 and s< 8, since the bounds prevent crossing the sign boundary.
PTR_TO_PACKETs with a variable offset part have an 'id', which is common to all
pointers sharing that same variable offset. This is important for packet range
checks: after adding some variable to a packet pointer, if you then copy it to
another register and (say) add a constant 4, both registers will share the same
'id' but one will have a fixed offset of +4. Then if it is bounds-checked and
found to be less than a PTR_TO_PACKET_END, the other register is now known to
have a safe range of at least 4 bytes. See 'Direct packet access', below, for
more on PTR_TO_PACKET ranges.
The 'id' field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of
the pointer returned from a map lookup. This means that when one copy is
checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs.
As well as range-checking, the tracked information is also used for enforcing
alignment of pointer accesses. For instance, on most systems the packet pointer
is 2 bytes after a 4-byte alignment. If a program adds 14 bytes to that to jump
over the Ethernet header, then reads IHL and addes (IHL * 4), the resulting
pointer will have a variable offset known to be 4n+2 for some n, so adding the 2
bytes (NET_IP_ALIGN) gives a 4-byte alignment and so word-sized accesses through
that pointer are safe.
Direct packet access
--------------------
In cls_bpf and act_bpf programs the verifier allows direct access to the packet
@ -1121,7 +1186,7 @@ it now points to 'skb->data + 14' and accessible range is [R5, R5 + 14 - 14)
which is zero bytes.
More complex packet access may look like:
R0=imm1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
R0=inv1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
6: r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */
7: r4 = *(u8 *)(r3 +12)
8: r4 *= 14
@ -1135,26 +1200,31 @@ More complex packet access may look like:
16: r2 += 8
17: r1 = *(u32 *)(r1 +80) /* load skb->data_end */
18: if r2 > r1 goto pc+2
R0=inv56 R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv52 R5=pkt(id=0,off=14,r=14) R10=fp
R0=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)) R5=pkt(id=0,off=14,r=14) R10=fp
19: r1 = *(u8 *)(r3 +4)
The state of the register R3 is R3=pkt(id=2,off=0,r=8)
id=2 means that two 'r3 += rX' instructions were seen, so r3 points to some
offset within a packet and since the program author did
'if (r3 + 8 > r1) goto err' at insn #18, the safe range is [R3, R3 + 8).
The verifier only allows 'add' operation on packet registers. Any other
operation will set the register state to 'unknown_value' and it won't be
The verifier only allows 'add'/'sub' operations on packet registers. Any other
operation will set the register state to 'SCALAR_VALUE' and it won't be
available for direct packet access.
Operation 'r3 += rX' may overflow and become less than original skb->data,
therefore the verifier has to prevent that. So it tracks the number of
upper zero bits in all 'uknown_value' registers, so when it sees
'r3 += rX' instruction and rX is more than 16-bit value, it will error as:
"cannot add integer value with N upper zero bits to ptr_to_packet"
therefore the verifier has to prevent that. So when it sees 'r3 += rX'
instruction and rX is more than 16-bit value, any subsequent bounds-check of r3
against skb->data_end will not give us 'range' information, so attempts to read
through the pointer will give "invalid access to packet" error.
Ex. after insn 'r4 = *(u8 *)(r3 +12)' (insn #7 above) the state of r4 is
R4=inv56 which means that upper 56 bits on the register are guaranteed
to be zero. After insn 'r4 *= 14' the state becomes R4=inv52, since
multiplying 8-bit value by constant 14 will keep upper 52 bits as zero.
Similarly 'r2 >>= 48' will make R2=inv48, since the shift is not sign
extending. This logic is implemented in evaluate_reg_alu() function.
R4=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) which means that upper 56 bits
of the register are guaranteed to be zero, and nothing is known about the lower
8 bits. After insn 'r4 *= 14' the state becomes
R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)), since multiplying an 8-bit
value by constant 14 will keep upper 52 bits as zero, also the least significant
bit will be zero as 14 is even. Similarly 'r2 >>= 48' will make
R2=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)), since the shift is not sign
extending. This logic is implemented in adjust_reg_min_max_vals() function,
which calls adjust_ptr_min_max_vals() for adding pointer to scalar (or vice
versa) and adjust_scalar_min_max_vals() for operations on two scalars.
The end result is that bpf program author can access packet directly
using normal C code as:
@ -1214,6 +1284,22 @@ The map is defined by:
. key size in bytes
. value size in bytes
Pruning
-------
The verifier does not actually walk all possible paths through the program. For
each new branch to analyse, the verifier looks at all the states it's previously
been in when at this instruction. If any of them contain the current state as a
subset, the branch is 'pruned' - that is, the fact that the previous state was
accepted implies the current state would be as well. For instance, if in the
previous state, r1 held a packet-pointer, and in the current state, r1 holds a
packet-pointer with a range as long or longer and at least as strict an
alignment, then r1 is safe. Similarly, if r2 was NOT_INIT before then it can't
have been used by any path from that point, so any value in r2 (including
another NOT_INIT) is safe. The implementation is in the function regsafe().
Pruning considers not only the registers but also the stack (and any spilled
registers it may hold). They must all be safe for the branch to be pruned.
This is implemented in states_equal().
Understanding eBPF verifier messages
------------------------------------