Re: [PATCH bpf-next 1/2] bpf: Fix check_stack_write_fixed_off() to correctly spill imm

Shung-Hsi Yu
2023-10-27T07:14:27Z

On Thu, Oct 26, 2023 at 05:13:10PM +0200, Hao Sun wrote:
> In check_stack_write_fixed_off(), imm value is cast to u32 before being
> spilled to the stack. Therefore, the sign information is lost, and the
> range information is incorrect when load from the stack again.
>
> For the following prog:
> 0: r2 = r10
> 1: *(u64*)(r2 -40) = -44
> 2: r0 = *(u64*)(r2 - 40)
> 3: if r0 s<= 0xa goto +2
> 4: r0 = 1
> 5: exit
> 6: r0 = 0
> 7: exit
>
> The verifier gives:
> func#0 @0
> 0: R1=ctx(off=0,imm=0) R10=fp0
> 0: (bf) r2 = r10 ; R2_w=fp0 R10=fp0
> 1: (7a) *(u64 *)(r2 -40) = -44 ; R2_w=fp0 fp-40_w=4294967252
> 2: (79) r0 = *(u64 *)(r2 -40) ; R0_w=4294967252 R2_w=fp0
> fp-40_w=4294967252
> 3: (c5) if r0 s< 0xa goto pc+2
> mark_precise: frame0: last_idx 3 first_idx 0 subseq_idx -1
> mark_precise: frame0: regs=r0 stack= before 2: (79) r0 = *(u64 *)(r2 -40)
> 3: R0_w=4294967252
> 4: (b7) r0 = 1 ; R0_w=1
> 5: (95) exit
> verification time 7971 usec
> stack depth 40
> processed 6 insns (limit 1000000) max_states_per_insn 0 total_states 0
> peak_states 0 mark_read 0
>
> So remove the incorrect cast, since imm field is declared as s32, and
> __mark_reg_known() takes u64, so imm would be correctly sign extended
> by compiler.
>
> Signed-off-by: Hao Sun <sunhao.th@gmail.com>

Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>

The acked-by applies to future version of the patchset as well.

FWIW I think we'd also need the same treatment for the (BPF_ALU | BPF_MOV |
BPF_K) case in check_alu_op().

> ---
> kernel/bpf/verifier.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 857d76694517..44af69ce1301 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4674,7 +4674,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> insn->imm != 0 && env->bpf_capable) {
> struct bpf_reg_state fake_reg = {};
>
> - __mark_reg_known(&fake_reg, (u32)insn->imm);
> + __mark_reg_known(&fake_reg, insn->imm);
> fake_reg.type = SCALAR_VALUE;
> save_register_state(state, spi, &fake_reg, size);
> } else if (reg && is_spillable_regtype(reg->type)) {
>
> --
> 2.34.1
>