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

On Wed, Jan 7, 2026 at 10:35 AM Lecomte, Arnaud [off-list ref] wrote:

On 07/01/2026 19:08, Lecomte, Arnaud wrote:

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On 06/01/2026 01:51, Andrii Nakryiko wrote:

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On Sun, Jan 4, 2026 at 12:52 PM Arnaud Lecomte
[off-list ref] wrote:

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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/ (local)
Fixes: e17d62fedd10 ("bpf: Refactor stack map trace depth
calculation into helper function")
Tested-by: syzbot+d1b7fa1092def3628bd7@syzkaller.appspotmail.com
Cc: Brahmajit Das <redacted>
Signed-off-by: Arnaud Lecomte <redacted>
---
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 | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index da3d328f5c15..e56752a9a891 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 {

@@ -479,7 +478,8 @@ static long __bpf_get_stack(struct pt_regs

*regs, struct task_struct *task,
                 goto err_fault;
         }

-       trace_nr = trace->nr - skip;
+       trace_nr = min(trace->nr, max_depth);

there is `trace->nr < skip` check right above, should it be moved here
and done against adjusted trace_nr (but before we subtract skip, of
course)?

We could indeed be more proactive on the clamping even-though I would
 say it does not fundamentally change anything in my opinion.
Happy to raise a new rev.

Nvm, this is not really possible as we are checking that the trace is
not NULL.
Moving it above could lead to a NULL dereference.

ok, so what are we doing then?

if (unlikely(!trace)) { ... }

trace_nr = min(trace->nr, max_depth);

if (trace->nr < skip) { ... }

trace_nr = trace->nr - skip;


(which is what I proposed, or am I still missing why this shouldn't be
done like that?)

pw-bot: cr

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+       trace_nr = trace_nr - skip;
         copy_len = trace_nr * elem_size;

         ips = trace->ip + skip;
--
2.43.0

Thanks for the review !
Arnaud