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

On Thu, Aug 20, 2026 at 8:25 AM Syeda Mahnur Asif <[email protected]> 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 <[email protected]>
> Signed-off-by: Syeda Mahnur Asif <[email protected]>
> ---
> 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?

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