syzbot reported a null-ptr-deref below [1] following a fault injection in
netfs_alloc_subrequest(). [0]

When netfs_alloc_subrequest() fails, subreq is NULL.
Later, netfs_prepare_write() tries to initialize members of
subreq(e.g., source), the issue in [1] is triggered.

Let's handle the error of netfs_prepare_write() properly.

[0]
FAULT_INJECTION: forcing a failure.
name failslab, interval 1, probability 0, space 0, times 0
Call Trace:
 netfs_alloc_subrequest+0x116/0x3f0
 netfs_prepare_write+0x76/0x7b0
 netfs_unbuffered_write+0x75c/0x2020
 netfs_unbuffered_write_iter_locked+0x7d6/0xa80
 netfs_unbuffered_write_iter+0x442/0x720
 v9fs_file_write_iter+0xbf/0x100
 vfs_write+0x6ac/0x1050

[1]
KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af]
RIP: 0010:netfs_prepare_write+0xbc/0x7b0 fs/netfs/write_issue.c:173
Call Trace:
 netfs_unbuffered_write+0x75c/0x2020 fs/netfs/direct_write.c:111
 netfs_unbuffered_write_iter_locked+0x7d6/0xa80 fs/netfs/direct_write.c:290
 netfs_unbuffered_write_iter+0x442/0x720 fs/netfs/direct_write.c:382
 v9fs_file_write_iter+0xbf/0x100 fs/9p/vfs_file.c:409
 new_sync_write fs/read_write.c:595 [inline]
 
Fixes: 288ace2f57c9 ("netfs: New writeback implementation")
Reported-by: syzbot+6a13fc77eb6f0802be2d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6a13fc77eb6f0802be2d
Tested-by: syzbot+6a13fc77eb6f0802be2d@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/netfs/direct_write.c | 4 ++++
 fs/netfs/write_issue.c  | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
index c16fbad286a1..567b946b7a56 100644
--- a/fs/netfs/direct_write.c
+++ b/fs/netfs/direct_write.c
@@ -110,6 +110,10 @@ static int netfs_unbuffered_write(struct netfs_io_request *wreq)
 		if (!subreq) {
 			netfs_prepare_write(wreq, stream, wreq->start + wreq->transferred);
 			subreq = stream->construct;
+			if (!subreq) {
+				wreq->error = -ENOMEM;
+				break;
+			}
 			stream->construct = NULL;
 		}
 
diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index 2d9cfcd43658..851f6f93ad45 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -170,6 +170,8 @@ void netfs_prepare_write(struct netfs_io_request *wreq,
 		rolling_buffer_make_space(&wreq->buffer, wreq->gfp);
 
 	subreq = netfs_alloc_subrequest(wreq);
+	if (!subreq)
+		return;
 	subreq->source		= stream->source;
 	subreq->start		= start;
 	subreq->stream_nr	= stream->stream_nr;
-- 
2.43.0