Re: [PATCH bpf v2] bpf: Fix NULL pointer dereference in bpf_sock_from_file

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Fri Aug 21, 2026 at 7:30 PM CEST, Andrii Nakryiko wrote:
> On Thu, Aug 20, 2026 at 8:25 AM Syeda Mahnur Asif <s.mahnur.a@gmail.com> wrote:
>>
>> bpf_sock_from_file should not dereference a NULL file pointer.
>> KASAN detects a null-ptr-deref when eBPF tracing
>> fentry/fexit programs are attached to points such as
>> __mmap_region and file_map_prot_check kernel functions. This can
>> result in a NULL file pointer flowing from context to the helper.
>>
>> A minimal check before dereferencing can fix this.
>>
>> Fixes: b60da4955f53 ("bpf: Only provide bpf_sock_from_file with CONFIG_NET")
>> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
>> Signed-off-by: Syeda Mahnur Asif <s.mahnur.a@gmail.com>
>> ---
>> v2:
>> - Added Reviewed-by from Emil Tsalapatis
>> - Fixed name in Signed-off-by
>>
>>  net/core/filter.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 16845987b244..7c3caae4bafc 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -12182,7 +12182,10 @@ const struct bpf_func_proto bpf_skc_to_mptcp_sock_proto = {
>>
>>  BPF_CALL_1(bpf_sock_from_file, struct file *, file)
>>  {
>> -       return (unsigned long)sock_from_file(file);
>> +       if (file)
>> +               return (unsigned long)sock_from_file(file);
>
> given this is a legacy ARG_PTR_TO_BTF_ID, it can be not just NULL, but
> also a) small value bogus "pointer" resulting from NULL + offset
> calculation and/or b) just random garbage casted to struct file
> pointer with bpf_core_cast() (maybe this hole we've closed with
> explicitly untrusted, not sure).
>
> Either way, this is not a sufficient fix at least.
>
> But I wonder if the proper fix is actually to mark this (and other
> similar) argument as explicitly requiring trusted PTR_TO_BTF_ID?
>
> Thoughts?
>

+1, should bite the bullet and accelerate deprecation of legacy PTR_TO_BTF_ID.
We can mark specific arguments / fields in structs as trusted where there is a
legitimate use case and the kernel guarantees their lifetime appropriately.

>> +
>> +       return (unsigned long)NULL;
>>  }
>>
>>  BTF_ID_LIST(bpf_sock_from_file_btf_ids)
>> --
>> 2.53.0
>>