linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Fix check_stack_write_fixed

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Date:   Fri, 27 Oct 2023 15:14:10 +0800
From:   Shung-Hsi Yu <shung-hsi.yu@...e.com>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        Eduard Zingerman <eddyz87@...il.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/2] bpf: Fix check_stack_write_fixed_off() to
 correctly spill imm

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@...il.com>

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