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

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

On 06/01/2026 01:51, Andrii Nakryiko wrote:
> On Sun, Jan 4, 2026 at 12:52 PM Arnaud Lecomte <contact@arnaud-lcm.com> 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>
>> ---
>> 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.
>> +       trace_nr = trace_nr - skip;
>>          copy_len = trace_nr * elem_size;
>>
>>          ips = trace->ip + skip;
>> --
>> 2.43.0
>>
Thanks for the review !
Arnaud