Re: [PATCH 01/26] netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry

Paulo Alcantara
2026-03-28T18:20:59Z

David Howells <dhowells@redhat.com> writes:

> From: Deepanshu Kartikey <kartikey406@gmail.com>
>
> When a write subrequest is marked NETFS_SREQ_NEED_RETRY, the retry path
> in netfs_unbuffered_write() unconditionally calls stream->prepare_write()
> without checking if it is NULL.
>
> Filesystems such as 9P do not set the prepare_write operation, so
> stream->prepare_write remains NULL. When get_user_pages() fails with
> -EFAULT and the subrequest is flagged for retry, this results in a NULL
> pointer dereference at fs/netfs/direct_write.c:189.
>
> Fix this by mirroring the pattern already used in write_retry.c: if
> stream->prepare_write is NULL, skip renegotiation and directly reissue
> the subrequest via netfs_reissue_write(), which handles iterator reset,
> IN_PROGRESS flag, stats update and reissue internally.
>
> Fixes: a0b4c7a49137 ("netfs: Fix unbuffered/DIO writes to dispatch subrequests in strict sequence")
> Reported-by: syzbot+7227db0fbac9f348dba0@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7227db0fbac9f348dba0
> Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
> fs/netfs/direct_write.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)

Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>