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.

Reported-by: syzbot+b18872ea9631b5dcef3b@syzkaller.appspotmail.com
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@gmail.com>
---
 net/core/skmsg.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index 4d75ef9d24bf..f5367356a483 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -432,6 +432,10 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
 			sge = sk_msg_elem(msg_rx, i);
 			copy = sge->length;
 			page = sg_page(sge);
+			if (!page) {
+				copied = copied ? copied : -EFAULT;
+				goto out;
+			}
 			if (copied + copy > len)
 				copy = len - copied;
 			copy = copy_page_to_iter(page, sge->offset, copy, iter);
-- 
2.49.0

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@syzkaller.appspotmail.com
> 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@gmail.com>

Does not apply to net. Please rebase and resend, adding the target tree
in the subj prefix and specifying a revision number.

Thanks,

Paolo

On Thu, Jul 17, 2025 at 4:40 PM Paolo Abeni <pabeni@redhat.com> 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@syzkaller.appspotmail.com
> > 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@gmail.com>
>
> 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

On Tue, Jul 15, 2025 at 01:41:58PM +0530, 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.
> 
> Reported-by: syzbot+b18872ea9631b5dcef3b@syzkaller.appspotmail.com
> 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@gmail.com>
> ---
>  net/core/skmsg.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 4d75ef9d24bf..f5367356a483 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -432,6 +432,10 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
>  			sge = sk_msg_elem(msg_rx, i);
>  			copy = sge->length;
>  			page = sg_page(sge);
> +			if (!page) {
> +				copied = copied ? copied : -EFAULT;
> +				goto out;
> +			}
>  			if (copied + copy > len)
>  				copy = len - copied;
>  			copy = copy_page_to_iter(page, sge->offset, copy, iter);
> -- 
> 2.49.0
> 
> 

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
a patch that has triggered this response.  He used to manually respond
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