linux-kernel - Re: [syzbot] KASAN: use-after-free Read in rxrpc_lookup

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Message-ID: <1265526.1670360970@warthog.procyon.org.uk>
Date:   Tue, 06 Dec 2022 21:09:30 +0000
From:   David Howells <dhowells@...hat.com>
To:     syzbot <syzbot+3538a6a72efa8b059c38@...kaller.appspotmail.com>
Cc:     dhowells@...hat.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, linux-afs@...ts.infradead.org,
        linux-kernel@...r.kernel.org, marc.dionne@...istor.com,
        netdev@...r.kernel.org, pabeni@...hat.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] KASAN: use-after-free Read in rxrpc_lookup_local

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git master

rxrpc: Fix NULL deref in rxrpc_unuse_local()

Fix rxrpc_unuse_local() to get the debug_id *after* checking to see if
local is NULL.

Fixes: a2cf3264f331 ("rxrpc: Fold __rxrpc_unuse_local() into rxrpc_unuse_local()")
Reported-by: syzbot+3538a6a72efa8b059c38@...kaller.appspotmail.com
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Marc Dionne <marc.dionne@...istor.com>
cc: linux-afs@...ts.infradead.org
---
 net/rxrpc/local_object.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)


diff --git a/net/rxrpc/local_object.c b/net/rxrpc/local_object.c
index 44222923c0d1..24ee585d9aaf 100644
--- a/net/rxrpc/local_object.c
+++ b/net/rxrpc/local_object.c
@@ -357,10 +357,11 @@ struct rxrpc_local *rxrpc_use_local(struct rxrpc_local *local,
  */
 void rxrpc_unuse_local(struct rxrpc_local *local, enum rxrpc_local_trace why)
 {
-	unsigned int debug_id = local->debug_id;
+	unsigned int debug_id;
 	int r, u;
 
 	if (local) {
+		debug_id = local->debug_id;
 		r = refcount_read(&local->ref);
 		u = atomic_dec_return(&local->active_users);
 		trace_rxrpc_local(debug_id, why, r, u);