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

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Message-ID: <6ce83e5c-de34-4ef2-b9f4-2ad15e645969@arnaud-lcm.com>
Date: Fri, 1 Aug 2025 19:16:47 +0100
From: "Lecomte, Arnaud" <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()

Well, it turns out it is less straightforward than it looked like to 
detect the memory corruption
  without KASAN. I am currently in holidays for the next 3 days so I've 
limited access to a
computer. I should be able to sort this out on monday.

Thanks,
Arnaud

On 30/07/2025 08:10, Arnaud Lecomte wrote:
> 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);
>>
>>