linux-kernel - Re: [PATCH v3 1/2] bpf: Skip scalar adjustment for BPF

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Message-ID: <0a2232a7faa9077ba7a837e066bd99bab812e4a6.camel@gmail.com>
Date: Wed, 01 Oct 2025 11:29:12 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: Brahmajit Das <listout@...tout.xyz>, 
	syzbot+d36d5ae81e1b0a53ef58@...kaller.appspotmail.com
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
 daniel@...earbox.net, 	haoluo@...gle.com, john.fastabend@...il.com,
 jolsa@...nel.org, kpsingh@...nel.org, 	linux-kernel@...r.kernel.org,
 martin.lau@...ux.dev, sdf@...ichev.me, 	song@...nel.org,
 syzkaller-bugs@...glegroups.com, yonghong.song@...ux.dev, 
	kafai.wan@...ux.dev
Subject: Re: [PATCH v3 1/2] bpf: Skip scalar adjustment for BPF_NEG if dst
 is a pointer

On Wed, 2025-10-01 at 15:26 +0530, Brahmajit Das wrote:
> In check_alu_op(), the verifier currently calls check_reg_arg() and
> adjust_scalar_min_max_vals() unconditionally for BPF_NEG operations.
> However, if the destination register holds a pointer, these scalar
> adjustments are unnecessary and potentially incorrect.
> 
> This patch adds a check to skip the adjustment logic when the destination
> register contains a pointer.
> 
> Reported-by: syzbot+d36d5ae81e1b0a53ef58@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=d36d5ae81e1b0a53ef58
> Fixes: aced132599b3 ("bpf: Add range tracking for BPF_NEG")
> Suggested-by: KaFai Wan <kafai.wan@...ux.dev>
> Signed-off-by: Brahmajit Das <listout@...tout.xyz>
> ---

Acked-by: Eduard Zingerman <eddyz87@...il.com>

>  kernel/bpf/verifier.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index e892df386eed..4b0924c38657 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -15645,7 +15645,8 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
>  		}
>  
>  		/* check dest operand */
> -		if (opcode == BPF_NEG) {
> +		if (opcode == BPF_NEG &&
> +		    !__is_pointer_value(false, &regs[insn->dst_reg])) {

Nit: I'd made this a bit simpler: `regs[insn->dst_reg].type == SCALAR_VALUE`,
     instead of __is_pointer_value() call.

>  			err = check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK);
>  			err = err ?: adjust_scalar_min_max_vals(env, insn,
>  							 &regs[insn->dst_reg],