Re: [syzbot] [netfs?] kernel BUG in netfs_limit_iter

[David Howells <dhowells@redhat.com>]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git c107785c7e8d

commit eb8299de8f603a6d7acf50e534c87ac1adeb3060
Author: Deepanshu Kartikey <kartikey406@gmail.com>
Date:   Sat Mar 7 10:09:47 2026 +0530

    netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry
    
    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>

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);