Re: [PATCH bpf-next v7 3/3] bpf: fix stackmap overflow check in __bpf_get_stackid()

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

On 05/09/2025 00:45, Song Liu wrote:
>
> On 9/3/25 4:43 PM, Arnaud Lecomte wrote:
>> Syzkaller reported a KASAN slab-out-of-bounds write in 
>> __bpf_get_stackid()
>> when copying stack trace data. The issue occurs when the perf trace
>>   contains more stack entries than the stack map bucket can hold,
>>   leading to an out-of-bounds write in the bucket's data array.
>>
>> Changes in v2:
>>   - Fixed max_depth names across get stack id
>>
>> Changes in v4:
>>   - Removed unnecessary empty line in __bpf_get_stackid
>>
>> Changs in v6:
>>   - Added back trace_len computation in __bpf_get_stackid
>>
>> Link to v6: 
>> https://lore.kernel.org/all/20250903135348.97884-1-contact@arnaud-lcm.com/
>>
>> Reported-by: syzbot+c9b724fbb41cf2538b7b@syzkaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=c9b724fbb41cf2538b7b
>> Fixes: ee2a098851bf ("bpf: Adjust BPF stack helper functions to 
>> accommodate skip > 0")
>> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
>> Acked-by: Yonghong Song <yonghong.song@linux.dev>
>
> For future patches, please keep the "Changes in vX.." at the end of
Good to know, thanks !
>
> your commit log and after a "---". IOW, something like
>
>
> Acked-by: Yonghong Song <yonghong.song@linux.dev>
>
> ---
>
> changes in v2:
>
> ...
>
> ---
>
> kernel/bpf/stackmap.c | 8 ++++++++
>
>
> In this way, the "changes in vXX" part will be removed by git-am.
>
>> ---
>>   kernel/bpf/stackmap.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 9f3ae426ddc3..29e05c9ff1bd 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -369,6 +369,7 @@ BPF_CALL_3(bpf_get_stackid_pe, struct 
>> bpf_perf_event_data_kern *, ctx,
>>   {
>>       struct perf_event *event = ctx->event;
>>       struct perf_callchain_entry *trace;
>> +    u32 elem_size, max_depth;
>>       bool kernel, user;
>>       __u64 nr_kernel;
>>       int ret;
>> @@ -390,11 +391,15 @@ BPF_CALL_3(bpf_get_stackid_pe, struct 
>> bpf_perf_event_data_kern *, ctx,
>>           return -EFAULT;
>>         nr_kernel = count_kernel_ip(trace);
>> +    elem_size = stack_map_data_size(map);
>>         if (kernel) {
>>           __u64 nr = trace->nr;
>>             trace->nr = nr_kernel;
>
> this trace->nr = is useless.
>
>> +        max_depth =
>> +            stack_map_calculate_max_depth(map->value_size, 
>> elem_size, flags);
>> +        trace->nr = min_t(u32, nr_kernel, max_depth);
>>           ret = __bpf_get_stackid(map, trace, flags);
>>             /* restore nr */
>> @@ -407,6 +412,9 @@ BPF_CALL_3(bpf_get_stackid_pe, struct 
>> bpf_perf_event_data_kern *, ctx,
>>               return -EFAULT;
>>             flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
>> +        max_depth =
>> +            stack_map_calculate_max_depth(map->value_size, 
>> elem_size, flags);
>> +        trace->nr = min_t(u32, trace->nr, max_depth);
>>           ret = __bpf_get_stackid(map, trace, flags);
>
> I missed this part earlier. Here we need to restore trace->nr, just 
> like we did

>
> in the "if (kernel)" branch.
>
Make sense, thanks !
> Thanks,
>
> Song
>
>>       }
>>       return ret;
>
Thanks,
Arnaud