linux-kernel - Re: [PATCH v2] bpf-next: Prevent out of bound buffer write in __bpf_get

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Message-Id: <f0c89037c4870388696751365cec002fe3ded78c7ef260bd99e1934ceb245acd@mail.kernel.org>
Date: Wed,  7 Jan 2026 18:31:38 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: contact@...aud-lcm.com,syzbot+d1b7fa1092def3628bd7@...kaller.appspotmail.com
Cc: andrii@...nel.org,ast@...nel.org,bpf@...r.kernel.org,contact@...aud-lcm.com,daniel@...earbox.net,eddyz87@...il.com,haoluo@...gle.com,john.fastabend@...il.com,jolsa@...nel.org,kpsingh@...nel.org,linux-kernel@...r.kernel.org,martin.lau@...ux.dev,netdev@...r.kernel.org,sdf@...ichev.me,song@...nel.org,syzkaller-bugs@...glegroups.com,yonghong.song@...ux.dev,listout@...tout.xyz,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH v2] bpf-next: Prevent out of bound buffer write in __bpf_get_stack

> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index da3d328f5c15..c0a430f9eafb 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -465,7 +465,6 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>
>  	if (trace_in) {
>  		trace = trace_in;
> -		trace->nr = min_t(u32, trace->nr, max_depth);
>  	} else if (kernel && task) {
>  		trace = get_callchain_entry_for_task(task, max_depth);
>  	} else {
> @@ -473,13 +472,15 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>  					   crosstask, false, 0);
>  	}
>
> +	trace_nr = min(trace->nr, max_depth);
                       ^^^^^^^^

Can this dereference trace when it is NULL? Both get_callchain_entry_for_task()
and get_perf_callchain() can return NULL, and several callers pass trace_in as
NULL (bpf_get_stack, bpf_get_stack_sleepable, __bpf_get_task_stack, and one
path in bpf_get_stack_pe).

The NULL check below comes after this dereference:

> +
> +	if (unlikely(!trace) || trace_nr < skip) {
                    ^^^^^^

This check is now ineffective because trace->nr was already dereferenced above.

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20791981532