linux-kernel - Re: [PATCH v2] bpf: fix stackmap overflow check in __bpf_get

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Message-ID: <5124b615-3a71-4a44-a497-eea3b5964fda@arnaud-lcm.com>
Date: Wed, 30 Jul 2025 08:10:58 +0100
From: Arnaud Lecomte <contact@...aud-lcm.com>
To: Yonghong Song <yonghong.song@...ux.dev>, song@...nel.org,
 jolsa@...nel.org, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev, eddyz87@...il.com, john.fastabend@...il.com,
 kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com,
 syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com
Subject: Re: [PATCH v2] bpf: fix stackmap overflow check in
 __bpf_get_stackid()

On 29/07/2025 23:45, Yonghong Song wrote:
>
>
> On 7/29/25 9:56 AM, 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.
>> For build_id mode, we use sizeof(struct bpf_stack_build_id)
>>   to determine capacity, and for normal mode we use sizeof(u64).
>>
>> Reported-by: syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=c9b724fbb41cf2538b7b
>> Tested-by: syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com
>> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
>
> Could you add a selftest? This way folks can easily find out what is
> the problem and why this fix solves the issue correctly.
>
Sure, will be done after work
Thanks,
Arnaud
>> ---
>> Changes in v2:
>>   - Use utilty stack_map_data_size to compute map stack map size
>> ---
>>   kernel/bpf/stackmap.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 3615c06b7dfa..6f225d477f07 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -230,7 +230,7 @@ static long __bpf_get_stackid(struct bpf_map *map,
>>       struct bpf_stack_map *smap = container_of(map, struct 
>> bpf_stack_map, map);
>>       struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
>>       u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
>> -    u32 hash, id, trace_nr, trace_len, i;
>> +    u32 hash, id, trace_nr, trace_len, i, max_depth;
>>       bool user = flags & BPF_F_USER_STACK;
>>       u64 *ips;
>>       bool hash_matches;
>> @@ -241,6 +241,12 @@ static long __bpf_get_stackid(struct bpf_map *map,
>>         trace_nr = trace->nr - skip;
>>       trace_len = trace_nr * sizeof(u64);
>> +
>> +    /* Clamp the trace to max allowed depth */
>> +    max_depth = smap->map.value_size / stack_map_data_size(map);
>> +    if (trace_nr > max_depth)
>> +        trace_nr = max_depth;
>> +
>>       ips = trace->ip + skip;
>>       hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0);
>>       id = hash & (smap->n_buckets - 1);
>
>