Re: [PATCH bpf 1/2] bpf, x86: Fix encoding for lower 8-bit registers in BPF_STX BPF_B

[Alexei Starovoitov]

On Sat, Apr 18, 2020 at 4:27 PM Luke Nelson <lukenels@xxxxxxxxxxxxxxxxx> wrote:
>
> This patch fixes an encoding bug in emit_stx for BPF_B when the source
> register is BPF_REG_FP.
>
> The current implementation for BPF_STX BPF_B in emit_stx saves one REX
> byte when the operands can be encoded using Mod-R/M alone. The lower 8
> bits of registers %rax, %rbx, %rcx, and %rdx can be accessed without using
> a REX prefix via %al, %bl, %cl, and %dl, respectively. Other registers,
> (e.g., %rsi, %rdi, %rbp, %rsp) require a REX prefix to use their 8-bit
> equivalents (%sil, %dil, %bpl, %spl).
>
> The current code checks if the source for BPF_STX BPF_B is BPF_REG_1
> or BPF_REG_2 (which map to %rdi and %rsi), in which case it emits the
> required REX prefix. However, it misses the case when the source is
> BPF_REG_FP (mapped to %rbp).
>
> The result is that BPF_STX BPF_B with BPF_REG_FP as the source operand
> will read from register %ch instead of the correct %bpl. This patch fixes
> the problem by fixing and refactoring the check on which registers need
> the extra REX byte. Since no BPF registers map to %rsp, there is no need
> to handle %spl.
>
> Fixes: 622582786c9e0 ("net: filter: x86: internal BPF JIT")
> Signed-off-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>

Applied. Thanks for the fix.
It's questionable whether the verifier should have allowed such insn
in the first place, but JIT fix is good regardless.