[D][PATCH] crypto: af_alg - fix use-after-free in af_alg_accept() due to bh_lock_sock()

[Mauricio Faria de Oliveira]

From: Herbert Xu <herbert at gondor.apana.org.au>

BugLink: https://bugs.launchpad.net/bugs/1884766

The locking in af_alg_release_parent is broken as the BH socket
lock can only be taken if there is a code-path to handle the case
where the lock is owned by process-context.  Instead of adding
such handling, we can fix this by changing the ref counts to
atomic_t.

This patch also modifies the main refcnt to include both normal
and nokey sockets.  This way we don't have to fudge the nokey
ref count when a socket changes from nokey to normal.

Credits go to Mauricio Faria de Oliveira who diagnosed this bug
and sent a patch for it:

https://lore.kernel.org/linux-crypto/20200605161657.535043-1-mfo@canonical.com/

Reported-by: Brian Moyles <bmoyles at netflix.com>
Reported-by: Mauricio Faria de Oliveira <mfo at canonical.com>
Fixes: 37f96694cf73 ("crypto: af_alg - Use bh_lock_sock in...")
Cc: <stable at vger.kernel.org>
Signed-off-by: Herbert Xu <herbert at gondor.apana.org.au>
(backported from commit 34c86f4c4a7be3b3e35aa48bd18299d4c756064d)
[mfo: backport: refresh context lines in af_alg.c hunk 4]
Signed-off-by: Mauricio Faria de Oliveira <mfo at canonical.com>
---
 crypto/af_alg.c         | 26 +++++++++++---------------
 crypto/algif_aead.c     |  9 +++------
 crypto/algif_hash.c     |  9 +++------
 crypto/algif_skcipher.c |  9 +++------
 include/crypto/if_alg.h |  4 ++--
 5 files changed, 22 insertions(+), 35 deletions(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index 4fc8e6a7abb2..272879d7b0d1 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -133,21 +133,15 @@ EXPORT_SYMBOL_GPL(af_alg_release);
 void af_alg_release_parent(struct sock *sk)
 {
 	struct alg_sock *ask = alg_sk(sk);
-	unsigned int nokey = ask->nokey_refcnt;
-	bool last = nokey && !ask->refcnt;
+	unsigned int nokey = atomic_read(&ask->nokey_refcnt);
 
 	sk = ask->parent;
 	ask = alg_sk(sk);
 
-	local_bh_disable();
-	bh_lock_sock(sk);
-	ask->nokey_refcnt -= nokey;
-	if (!last)
-		last = !--ask->refcnt;
-	bh_unlock_sock(sk);
-	local_bh_enable();
+	if (nokey)
+		atomic_dec(&ask->nokey_refcnt);
 
-	if (last)
+	if (atomic_dec_and_test(&ask->refcnt))
 		sock_put(sk);
 }
 EXPORT_SYMBOL_GPL(af_alg_release_parent);
@@ -192,7 +186,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 
 	err = -EBUSY;
 	lock_sock(sk);
-	if (ask->refcnt | ask->nokey_refcnt)
+	if (atomic_read(&ask->refcnt))
 		goto unlock;
 
 	swap(ask->type, type);
@@ -241,7 +235,7 @@ static int alg_setsockopt(struct socket *sock, int level, int optname,
 	int err = -EBUSY;
 
 	lock_sock(sk);
-	if (ask->refcnt)
+	if (atomic_read(&ask->refcnt) != atomic_read(&ask->nokey_refcnt))
 		goto unlock;
 
 	type = ask->type;
@@ -308,12 +302,14 @@ int af_alg_accept(struct sock *sk, struct socket *newsock, bool kern)
 
 	sk2->sk_family = PF_ALG;
 
-	if (nokey || !ask->refcnt++)
+	if (atomic_inc_return_relaxed(&ask->refcnt) == 1)
 		sock_hold(sk);
-	ask->nokey_refcnt += nokey;
+	if (nokey) {
+		atomic_inc(&ask->nokey_refcnt);
+		atomic_set(&alg_sk(sk2)->nokey_refcnt, 1);
+	}
 	alg_sk(sk2)->parent = sk;
 	alg_sk(sk2)->type = type;
-	alg_sk(sk2)->nokey_refcnt = nokey;
 
 	newsock->ops = type->ops;
 	newsock->state = SS_CONNECTED;
diff --git a/crypto/algif_aead.c b/crypto/algif_aead.c
index eb100a04ce9f..e643d268c7ba 100644
--- a/crypto/algif_aead.c
+++ b/crypto/algif_aead.c
@@ -388,7 +388,7 @@ static int aead_check_key(struct socket *sock)
 	struct alg_sock *ask = alg_sk(sk);
 
 	lock_sock(sk);
-	if (ask->refcnt)
+	if (!atomic_read(&ask->nokey_refcnt))
 		goto unlock_child;
 
 	psk = ask->parent;
@@ -400,11 +400,8 @@ static int aead_check_key(struct socket *sock)
 	if (crypto_aead_get_flags(tfm->aead) & CRYPTO_TFM_NEED_KEY)
 		goto unlock;
 
-	if (!pask->refcnt++)
-		sock_hold(psk);
-
-	ask->refcnt = 1;
-	sock_put(psk);
+	atomic_dec(&pask->nokey_refcnt);
+	atomic_set(&ask->nokey_refcnt, 0);
 
 	err = 0;
 
diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
index d0cde541beb6..73abe6a33f3a 100644
--- a/crypto/algif_hash.c
+++ b/crypto/algif_hash.c
@@ -306,7 +306,7 @@ static int hash_check_key(struct socket *sock)
 	struct alg_sock *ask = alg_sk(sk);
 
 	lock_sock(sk);
-	if (ask->refcnt)
+	if (!atomic_read(&ask->nokey_refcnt))
 		goto unlock_child;
 
 	psk = ask->parent;
@@ -318,11 +318,8 @@ static int hash_check_key(struct socket *sock)
 	if (crypto_ahash_get_flags(tfm) & CRYPTO_TFM_NEED_KEY)
 		goto unlock;
 
-	if (!pask->refcnt++)
-		sock_hold(psk);
-
-	ask->refcnt = 1;
-	sock_put(psk);
+	atomic_dec(&pask->nokey_refcnt);
+	atomic_set(&ask->nokey_refcnt, 0);
 
 	err = 0;
 
diff --git a/crypto/algif_skcipher.c b/crypto/algif_skcipher.c
index cfdaab2b7d76..75444b8beed9 100644
--- a/crypto/algif_skcipher.c
+++ b/crypto/algif_skcipher.c
@@ -219,7 +219,7 @@ static int skcipher_check_key(struct socket *sock)
 	struct alg_sock *ask = alg_sk(sk);
 
 	lock_sock(sk);
-	if (ask->refcnt)
+	if (!atomic_read(&ask->nokey_refcnt))
 		goto unlock_child;
 
 	psk = ask->parent;
@@ -231,11 +231,8 @@ static int skcipher_check_key(struct socket *sock)
 	if (crypto_skcipher_get_flags(tfm) & CRYPTO_TFM_NEED_KEY)
 		goto unlock;
 
-	if (!pask->refcnt++)
-		sock_hold(psk);
-
-	ask->refcnt = 1;
-	sock_put(psk);
+	atomic_dec(&pask->nokey_refcnt);
+	atomic_set(&ask->nokey_refcnt, 0);
 
 	err = 0;
 
diff --git a/include/crypto/if_alg.h b/include/crypto/if_alg.h
index 482461d8931d..11f107df78dc 100644
--- a/include/crypto/if_alg.h
+++ b/include/crypto/if_alg.h
@@ -34,8 +34,8 @@ struct alg_sock {
 
 	struct sock *parent;
 
-	unsigned int refcnt;
-	unsigned int nokey_refcnt;
+	atomic_t refcnt;
+	atomic_t nokey_refcnt;
 
 	const struct af_alg_type *type;
 	void *private;
-- 
2.25.1