[PATCH] ceph: require the dentry lease record to cover struct ceph_mds_reply_lease

[Xiang Mei]

parse_reply_info_lease() reads a 32-bit struct_len off the wire and only
checks that many bytes are present, without requiring struct_len to cover
sizeof(struct ceph_mds_reply_lease) (10 bytes).  A malicious MDS can
declare a shorter record and still get the pointer published: the
following *p += sizeof(**lease) overshoot is undone by *p = lend, so
nothing downstream rejects the reply.  ceph_fill_trace() ->
update_dentry_lease() then reads 10 bytes through that pointer on any
lookup on a mounted CephFS, running past the end of the message front.

Require the record to cover the structure, matching the sibling
parse_reply_info_dir().  A conforming MDS always encodes the full 10
bytes.

  BUG: KASAN: slab-out-of-bounds in __update_dentry_lease.constprop.0 (fs/ceph/inode.c:1459)
  Read of size 4 at addr ffff888026c63d65 by task kworker/0:3/2366
  Workqueue: ceph-msgr ceph_con_workfn
  Call Trace:
   __update_dentry_lease.constprop.0 (fs/ceph/inode.c:1459)
   ceph_fill_trace (fs/ceph/inode.c:1474 fs/ceph/inode.c:1823)
   mds_dispatch (fs/ceph/mds_client.c:4229 fs/ceph/mds_client.c:7225)
   ceph_con_process_message (net/ceph/messenger.c:1424)
   ceph_con_v1_try_read (net/ceph/messenger_v1.c:1430)
   ceph_con_workfn (net/ceph/messenger.c:1576)
   process_one_work (kernel/workqueue.c:3396)
   worker_thread (kernel/workqueue.c:3479 kernel/workqueue.c:3560)
   kthread (kernel/kthread.c:436)
   ret_from_fork (arch/x86/kernel/process.c:158)
   ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

  The buggy address is located 357 bytes inside of
   allocated 360-byte region [ffff888026c63c00, ffff888026c63d68)

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 4ac4c23eaa38 ("ceph: decode alternate_name in lease info")
Reported-by: co+f92fbe44e3c6df69@xxxxxxx
Assisted-by: LLM
Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
---
 fs/ceph/mds_client.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 085ae0cfb5f7..073d9f4a3adb 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -380,6 +380,8 @@ static int parse_reply_info_lease(void **p, void *end,
 
 	lend = *p + struct_len;
 	ceph_decode_need(p, end, struct_len, bad);
+	if (struct_len < sizeof(**lease))
+		goto bad;
 	*lease = *p;
 	*p += sizeof(**lease);
 
-- 
2.43.0