[PATCH 01/11] netfs: Fix early put of sink folio in netfs_read_gaps()

David Howells
2026-04-20T08:37:27Z

Fix netfs_read_gaps() to release the sink page it uses after waiting for
the request to complete. The way the sink page is used is that an
ITER_BVEC-class iterator is created that has the gaps from the target folio
at either end, but has the sink page tiled over the middle so that a single
read op can fill in both gaps.

The bug was found by KASAN detecting a UAF on the generic/075 xfstest in
the cifsd kernel thread that handles reception of data from the TCP socket:

BUG: KASAN: use-after-free in _copy_to_iter+0x48a/0xa20
Write of size 885 at addr ffff888107f92000 by task cifsd/1285
CPU: 2 UID: 0 PID: 1285 Comm: cifsd Not tainted 7.0.0 #6 PREEMPT(lazy)
Call Trace:
dump_stack_lvl+0x5d/0x80
print_report+0x17f/0x4f1
kasan_report+0x100/0x1e0
kasan_check_range+0x10f/0x1e0
__asan_memcpy+0x3c/0x60
_copy_to_iter+0x48a/0xa20
__skb_datagram_iter+0x2c9/0x430
skb_copy_datagram_iter+0x6e/0x160
tcp_recvmsg_locked+0xce0/0x1130
tcp_recvmsg+0xeb/0x300
inet_recvmsg+0xcf/0x3a0
sock_recvmsg+0xea/0x100
cifs_readv_from_socket+0x3a6/0x4d0 [cifs]
cifs_read_iter_from_socket+0xdd/0x130 [cifs]
cifs_readv_receive+0xaad/0xb10 [cifs]
cifs_demultiplex_thread+0x1148/0x1740 [cifs]
kthread+0x1cf/0x210

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Reported-by: Steve French <sfrench@samba.org>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
fs/netfs/buffered_read.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index c23173a061e8..dc50690fdd44 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -459,9 +459,6 @@ static int netfs_read_gaps(struct file *file, struct folio *folio)

netfs_read_to_pagecache(rreq, NULL);

- if (sink)
- folio_put(sink);
-
ret = netfs_wait_for_read(rreq);
if (ret >= 0) {
folio_detach_private(folio);
@@ -471,6 +468,9 @@ static int netfs_read_gaps(struct file *file, struct folio *folio)
flush_dcache_folio(folio);
folio_mark_uptodate(folio);
}
+
+ if (sink)
+ folio_put(sink);
folio_unlock(folio);
netfs_put_request(rreq, netfs_rreq_trace_put_return);
return ret < 0 ? ret : 0;