Re: [PATCH bpf v2 2/2] bpf, x86_32: Fix clobbering of dst for BPF_JSET

[Wang YanQing]

On Wed, Apr 22, 2020 at 10:36:30AM -0700, Luke Nelson wrote:
> The current JIT clobbers the destination register for BPF_JSET BPF_X
> and BPF_K by using "and" and "or" instructions. This is fine when the
> destination register is a temporary loaded from a register stored on
> the stack but not otherwise.
> 
> This patch fixes the problem (for both BPF_K and BPF_X) by always loading
> the destination register into temporaries since BPF_JSET should not
> modify the destination register.
> 
> This bug may not be currently triggerable as BPF_REG_AX is the only
> register not stored on the stack and the verifier uses it in a limited
> way.
> 
> Fixes: 03f5781be2c7b ("bpf, x86_32: add eBPF JIT compiler for ia32")
> Signed-off-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>
Acked-by: Wang YanQing <udknight@xxxxxxxxx>