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

["Lecomte, Arnaud" <contact@arnaud-lcm.com>]

Aborting in favor of my comment on the first rev.

On 07/01/2026 19:12, Arnaud Lecomte wrote:
> Syzkaller reported a KASAN slab-out-of-bounds write in __bpf_get_stack()
> during stack trace copying.
>
> The issue occurs when: the callchain entry (stored as a per-cpu variable)
> grow between collection and buffer copy, causing it to exceed the initially
> calculated buffer size based on max_depth.
>
> The callchain collection intentionally avoids locking for performance
> reasons, but this creates a window where concurrent modifications can
> occur during the copy operation.
>
> To prevent this from happening, we clamp the trace len to the max
> depth initially calculated with the buffer size and the size of
> a trace.
>
> Reported-by: syzbot+d1b7fa1092def3628bd7@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/691231dc.a70a0220.22f260.0101.GAE@google.com/T/
> Fixes: e17d62fedd10 ("bpf: Refactor stack map trace depth calculation into helper function")
> Tested-by: syzbot+d1b7fa1092def3628bd7@syzkaller.appspotmail.com
> Cc: Brahmajit Das <listout@listout.xyz>
> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
> ---
> Changes in v2:
> 	- Moved the trace_nr clamping to max_depth above trace->nr skip
> 	  verification.
> Link to v1: https://lore.kernel.org/all/20260104205220.980752-1-contact@arnaud-lcm.com/
>
> Thanks Brahmajit Das for the initial fix he proposed that I tweaked
> with the correct justification and a better implementation in my
> opinion.
> ---
>   kernel/bpf/stackmap.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> 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);
>   	}
>   
> -	if (unlikely(!trace) || trace->nr < skip) {
> +	trace_nr = min(trace->nr, max_depth);
> +
> +	if (unlikely(!trace) || trace_nr < skip) {
>   		if (may_fault)
>   			rcu_read_unlock();
>   		goto err_fault;
>   	}
>   
> -	trace_nr = trace->nr - skip;
> +	trace_nr = trace_nr - skip;
>   	copy_len = trace_nr * elem_size;
>   
>   	ips = trace->ip + skip;