[PATCH v2] cifs: fix use-after-free of server info on cifs_ses_add_channel error

[Henry Martin]

The error path of cifs_ses_add_channel() calls cifs_put_tcp_session()
before cifs_chan_clear_need_reconnect().  The latter reaches
cifs_ses_get_chan_index(), which dereferences server->terminate; if
the put was the final reference, TCP_Server_Info is already freed and
this is a use-after-free.  Same put-then-use pattern as the recently
fixed sibling in commit 717e0a25036b ("cifs: Fix server use-after-free
in cifs_chan_skip_or_disable()").

Move the put last, but do not read chan->server after dropping
chan_lock: chan points into ses->chans[], and once chan_count is
decremented a concurrent add_channel can reuse the slot and overwrite
the pointer, so the put would act on the new channel's server.  Cache
the pointer under chan_lock, clear the reconnect bit and decrement
chan_count while still holding the reference, and release it after
the unlock.

This issue was discovered by Tencent CodeBuddy Security.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: ee1d21794e55a ("cifs: handle when server stops supporting multichannel")
Signed-off-by: Henry Martin <bsdhenrymartin@xxxxxxxxx>
---

v2: Cache chan->server in a local under chan_lock and use the cached
    pointer for cifs_put_tcp_session(); the slot can be reused by a
    concurrent add once chan_count is decremented.

 fs/smb/client/sess.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
index e095f41b58828..ba0b4057322c8 100644
--- a/fs/smb/client/sess.c
+++ b/fs/smb/client/sess.c
@@ -626,12 +626,19 @@ cifs_ses_add_channel(struct cifs_ses *ses,
 
 out:
 	if (rc && chan->server) {
-		cifs_put_tcp_session(chan->server, 0);
+		struct TCP_Server_Info *pserver;
 
 		spin_lock(&ses->chan_lock);
 
+		/*
+		 * Cache the server pointer while holding chan_lock: once
+		 * chan_count is decremented below, this slot can be reused
+		 * by a concurrent add_channel and chan->server overwritten.
+		 */
+		pserver = chan->server;
+
 		/* we rely on all bits beyond chan_count to be clear */
-		cifs_chan_clear_need_reconnect(ses, chan->server);
+		cifs_chan_clear_need_reconnect(ses, pserver);
 		ses->chan_count--;
 		/*
 		 * chan_count should never reach 0 as at least the primary
@@ -639,6 +646,8 @@ cifs_ses_add_channel(struct cifs_ses *ses,
 		 */
 		WARN_ON(ses->chan_count < 1);
 		spin_unlock(&ses->chan_lock);
+
+		cifs_put_tcp_session(pserver, 0);
 	}
 
 	kfree(ctx->UNC);
-- 
2.43.7