[PATCH] crypto: null - Use spin lock instead of mutex

[Herbert Xu]

On Mon, Dec 30, 2024 at 11:22:28AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    a024e377efed net: llc: reset skb->transport_header
> git tree:       net
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=15c7f0b0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6a2b862bf4a5409f
> dashboard link: https://syzkaller.appspot.com/bug?extid=b3e02953598f447d4d2a
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14bce818580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12bce818580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/f2ea524d69fe/disk-a024e377.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/b39d227b097d/vmlinux-a024e377.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/8ee66636253f/bzImage-a024e377.xz

---8<---
As the null algorithm may be freed in softirq context through
af_alg, use spin locks instead of mutexes to protect the default
null algorithm.

Reported-by: syzbot+b3e02953598f447d4d2a@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

diff --git a/crypto/crypto_null.c b/crypto/crypto_null.c
index 5b84b0f7cc17..337867028653 100644
--- a/crypto/crypto_null.c
+++ b/crypto/crypto_null.c
@@ -17,10 +17,10 @@
 #include <crypto/internal/skcipher.h>
 #include <linux/init.h>
 #include <linux/module.h>
-#include <linux/mm.h>
+#include <linux/spinlock.h>
 #include <linux/string.h>
 
-static DEFINE_MUTEX(crypto_default_null_skcipher_lock);
+static DEFINE_SPINLOCK(crypto_default_null_skcipher_lock);
 static struct crypto_sync_skcipher *crypto_default_null_skcipher;
 static int crypto_default_null_skcipher_refcnt;
 
@@ -152,23 +152,32 @@ MODULE_ALIAS_CRYPTO("cipher_null");
 
 struct crypto_sync_skcipher *crypto_get_default_null_skcipher(void)
 {
+	struct crypto_sync_skcipher *ntfm = NULL;
 	struct crypto_sync_skcipher *tfm;
 
-	mutex_lock(&crypto_default_null_skcipher_lock);
+	spin_lock_bh(&crypto_default_null_skcipher_lock);
 	tfm = crypto_default_null_skcipher;
 
 	if (!tfm) {
-		tfm = crypto_alloc_sync_skcipher("ecb(cipher_null)", 0, 0);
-		if (IS_ERR(tfm))
-			goto unlock;
+		spin_unlock_bh(&crypto_default_null_skcipher_lock);
 
-		crypto_default_null_skcipher = tfm;
+		ntfm = crypto_alloc_sync_skcipher("ecb(cipher_null)", 0, 0);
+		if (IS_ERR(ntfm))
+			return ntfm;
+
+		spin_lock_bh(&crypto_default_null_skcipher_lock);
+		tfm = crypto_default_null_skcipher;
+		if (!tfm) {
+			tfm = ntfm;
+			ntfm = NULL;
+			crypto_default_null_skcipher = tfm;
+		}
 	}
 
 	crypto_default_null_skcipher_refcnt++;
+	spin_unlock_bh(&crypto_default_null_skcipher_lock);
 
-unlock:
-	mutex_unlock(&crypto_default_null_skcipher_lock);
+	crypto_free_sync_skcipher(ntfm);
 
 	return tfm;
 }
@@ -176,12 +185,16 @@ EXPORT_SYMBOL_GPL(crypto_get_default_null_skcipher);
 
 void crypto_put_default_null_skcipher(void)
 {
-	mutex_lock(&crypto_default_null_skcipher_lock);
+	struct crypto_sync_skcipher *tfm = NULL;
+
+	spin_lock_bh(&crypto_default_null_skcipher_lock);
 	if (!--crypto_default_null_skcipher_refcnt) {
-		crypto_free_sync_skcipher(crypto_default_null_skcipher);
+		tfm = crypto_default_null_skcipher;
 		crypto_default_null_skcipher = NULL;
 	}
-	mutex_unlock(&crypto_default_null_skcipher_lock);
+	spin_unlock_bh(&crypto_default_null_skcipher_lock);
+
+	crypto_free_sync_skcipher(tfm);
 }
 EXPORT_SYMBOL_GPL(crypto_put_default_null_skcipher);
 
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt