[PATCH] ksmbd: only rebind the reopened file's own oplock on durable reconnect

From: Aldo Ariel Panzardo <[email protected]>

ksmbd_reopen_durable_fd() walks the inode's m_op_list and rebinds every
detached oplock to the reconnecting session:

	list_for_each_entry_rcu(op, &ci->m_op_list, op_entry,
				lockdep_is_held(&ci->m_lock)) {
		if (op->conn)
			continue;
		op->conn = ksmbd_conn_get(fp->conn);
		op->sess = work->sess;
	}

The only key is op->conn == NULL, which every detached durable handle on
that inode matches, not just the one owned by fp.  When two sessions hold
durable handles on the same file and both disconnect, reconnecting one of
them adopts the other session's oplock: op->sess is overwritten with the
reconnecting session without taking a reference on it, while op->conn
pins the connection.

The sibling teardown path, session_fd_check(), keys on the identity of
the connection being torn down (op->conn == conn) rather than on shared
state, and so does not have this problem.

Once the adopting session is destroyed, ksmbd_session_destroy() frees it
while the foreign oplock still points at it.  The reader in
ksmbd_close_fd_app_instance_id() validates only opinfo->conn, which is
still live thanks to the reference taken above, and then dereferences the
stale session:

	if (!opinfo->conn) {
		up_read(&fp->f_ci->m_lock);
		goto out;
	}

	ft = &opinfo->sess->file_table;
	write_lock(&ft->lock);

  BUG: KASAN: slab-use-after-free in _raw_write_lock+0x74/0xd0
  Write of size 4 at addr ffff88810a970528 by task kworker/0:0/9
  Workqueue: ksmbd-io handle_ksmbd_work
  Call Trace:
   _raw_write_lock+0x74/0xd0
   ksmbd_close_fd_app_instance_id+0x183/0x410
   smb2_open+0x1346/0x4430
   handle_ksmbd_work+0x2bb/0x7b0

Reached from an authenticated session against a share with the default
durable-handle and oplock configuration: two sessions open the same file
with a durable-v2 handle and an RH lease under distinct AppInstanceIds,
both log off, one reconnects with DH2C, and a later durable-v2 create
carrying the other AppInstanceId walks into the freed session.

Constrain the loop to the oplock owned by the file being reopened.

Fixes: f363a0fb134a ("ksmbd: fix app-instance durable supersede session UAF")
Cc: [email protected]
Signed-off-by: Aldo Ariel Panzardo <[email protected]>
---
 fs/smb/server/vfs_cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/smb/server/vfs_cache.c b/fs/smb/server/vfs_cache.c
index d95c405eab11..1e9d13344825 100644
--- a/fs/smb/server/vfs_cache.c
+++ b/fs/smb/server/vfs_cache.c
@@ -1720,7 +1720,7 @@ int ksmbd_reopen_durable_fd(struct ksmbd_work *work, struct ksmbd_file *fp)
 	down_write(&ci->m_lock);
 	list_for_each_entry_rcu(op, &ci->m_op_list, op_entry,
 				lockdep_is_held(&ci->m_lock)) {
-		if (op->conn)
+		if (op->conn || op->o_fp != fp)
 			continue;
 		op->conn = ksmbd_conn_get(fp->conn);
 		op->sess = work->sess;
-- 
2.43.0