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

David Howells
2026-03-26T10:46:25Z

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(-)

diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
index dd1451bf7543..4d9760e36c11 100644
--- a/fs/netfs/direct_write.c
+++ b/fs/netfs/direct_write.c
@@ -186,10 +186,18 @@ static int netfs_unbuffered_write(struct netfs_io_request *wreq)
stream->sreq_max_segs = INT_MAX;

netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);
- stream->prepare_write(subreq);

- __set_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags);
- netfs_stat(&netfs_n_wh_retry_write_subreq);
+ if (stream->prepare_write) {
+ stream->prepare_write(subreq);
+ __set_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags);
+ netfs_stat(&netfs_n_wh_retry_write_subreq);
+ } else {
+ struct iov_iter source;
+
+ netfs_reset_iter(subreq);
+ source = subreq->io_iter;
+ netfs_reissue_write(stream, subreq, &source);
+ }
}

netfs_unbuffered_write_done(wreq);