Re: [PATCH] net: skmsg: fix NULL pointer dereference in sk_msg_recvmsg()

[Pranav Tyagi]

On Thu, Jul 17, 2025 at 4:40 PM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
>
> On 7/15/25 10:11 AM, Pranav Tyagi wrote:
> > A NULL page from sg_page() in sk_msg_recvmsg() can reach
> > __kmap_local_page_prot() and crash the kernel. Add a check for the page
> > before calling copy_page_to_iter() and fail early with -EFAULT to
> > prevent the crash.
>
> Interesting. I thought the sge in this case are build from the kernel, I
> did not expect a null page to be possible. Can you describe in the
> commit message how such bad sges are created?
>
> >
> > Reported-by: syzbot+b18872ea9631b5dcef3b@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=b18872ea9631b5dcef3b
> > Fixes: 2bc793e3272a ("skmsg: Extract __tcp_bpf_recvmsg() and tcp_bpf_wait_data()")
> > Signed-off-by: Pranav Tyagi <pranav.tyagi03@xxxxxxxxx>
>
> Does not apply to net. Please rebase and resend, adding the target tree
> in the subj prefix and specifying a revision number.
>
> Thanks,
>
> Paolo
>
>

Hi Paolo,

Apologies for a delayed response. I will send a v2 of the patch with all the
required changes shortly. Also, as I am a beginner at kernel work, my initial
intention was to just fix the bug and (honestly) I did not think about
the cause of
bad sges. But, I will definitely take a deep dive and try my level best
to look for a suitable explanation for the same.

Thanks for the feedback.

Regards
Pranav Tyagi