Puranjay Mohan b599d7d26d bpf, x86: Fix PROBE_MEM runtime load check ... When a load is marked PROBE_MEM - e.g. due to PTR_UNTRUSTED access - the address being loaded from is not necessarily valid. The BPF jit sets up exception handlers for each such load which catch page faults and 0 out the destination register. If the address for the load is outside kernel address space, the load will escape the exception handling and crash the kernel. To prevent this from happening, the emits some instruction to verify that addr is > end of userspace addresses. x86 has a legacy vsyscall ABI where a page at address 0xffffffffff600000 is mapped with user accessible permissions. The addresses in this page are considered userspace addresses by the fault handler. Therefore, a BPF program accessing this page will crash the kernel. This patch fixes the runtime checks to also check that the PROBE_MEM address is below VSYSCALL_ADDR. Example BPF program: SEC("fentry/tcp_v4_connect") int BPF_PROG(fentry_tcp_v4_connect, struct sock *sk) { *(volatile unsigned long *)&sk->sk_tsq_flags; return 0; } BPF Assembly: 0: (79) r1 = *(u64 *)(r1 +0) 1: (79) r1 = *(u64 *)(r1 +344) 2: (b7) r0 = 0 3: (95) exit x86-64 JIT ========== BEFORE AFTER ------ ----- 0: nopl 0x0(%rax,%rax,1) 0: nopl 0x0(%rax,%rax,1) 5: xchg %ax,%ax 5: xchg %ax,%ax 7: push %rbp 7: push %rbp 8: mov %rsp,%rbp 8: mov %rsp,%rbp b: mov 0x0(%rdi),%rdi b: mov 0x0(%rdi),%rdi ------------------------------------------------------------------------------- f: movabs $0x100000000000000,%r11 f: movabs $0xffffffffff600000,%r10 19: add $0x2a0,%rdi 19: mov %rdi,%r11 20: cmp %r11,%rdi 1c: add $0x2a0,%r11 23: jae 0x0000000000000029 23: sub %r10,%r11 25: xor %edi,%edi 26: movabs $0x100000000a00000,%r10 27: jmp 0x000000000000002d 30: cmp %r10,%r11 29: mov 0x0(%rdi),%rdi 33: ja 0x0000000000000039 --------------------------------\ 35: xor %edi,%edi 2d: xor %eax,%eax \ 37: jmp 0x0000000000000040 2f: leave \ 39: mov 0x2a0(%rdi),%rdi 30: ret \-------------------------------------------- 40: xor %eax,%eax 42: leave 43: ret Signed-off-by: Puranjay Mohan <puranjay@kernel.org> Link: https://lore.kernel.org/r/20240424100210.11982-3-puranjay@kernel.org Signed-off-by: Alexei Starovoitov <ast@kernel.org>		2024-04-26 09:45:18 -07:00
..
bpf_jit_comp32.c	x86/bugs: Rename CONFIG_RETPOLINE => CONFIG_MITIGATION_RETPOLINE	2024-01-10 10:52:28 +01:00
bpf_jit_comp.c	bpf, x86: Fix PROBE_MEM runtime load check	2024-04-26 09:45:18 -07:00
Makefile	treewide: Add SPDX license identifier - Makefile/Kconfig	2019-05-21 10:50:46 +02:00