linux-kernel - [PATCH 3.16 18/99] crypto: api - Fix race condition in crypto_spawn

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Message-ID: <lsq.1589984008.725547610@decadent.org.uk>
Date:   Wed, 20 May 2020 15:13:46 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Herbert Xu" <herbert@...dor.apana.org.au>
Subject: [PATCH 3.16 18/99] crypto: api - Fix race condition in
 crypto_spawn_alg

3.16.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Herbert Xu <herbert@...dor.apana.org.au>

commit 73669cc556462f4e50376538d77ee312142e8a8a upstream.

The function crypto_spawn_alg is racy because it drops the lock
before shooting the dying algorithm.  The algorithm could disappear
altogether before we shoot it.

This patch fixes it by moving the shooting into the locked section.

Fixes: 6bfd48096ff8 ("[CRYPTO] api: Added spawns")
Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 crypto/algapi.c   | 16 +++++-----------
 crypto/api.c      |  3 +--
 crypto/internal.h |  1 -
 3 files changed, 6 insertions(+), 14 deletions(-)

--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -628,22 +628,16 @@ EXPORT_SYMBOL_GPL(crypto_drop_spawn);
 static struct crypto_alg *crypto_spawn_alg(struct crypto_spawn *spawn)
 {
 	struct crypto_alg *alg;
-	struct crypto_alg *alg2;
 
 	down_read(&crypto_alg_sem);
 	alg = spawn->alg;
-	alg2 = alg;
-	if (alg2)
-		alg2 = crypto_mod_get(alg2);
-	up_read(&crypto_alg_sem);
-
-	if (!alg2) {
-		if (alg)
-			crypto_shoot_alg(alg);
-		return ERR_PTR(-EAGAIN);
+	if (alg && !crypto_mod_get(alg)) {
+		alg->cra_flags |= CRYPTO_ALG_DYING;
+		alg = NULL;
 	}
+	up_read(&crypto_alg_sem);
 
-	return alg;
+	return alg ?: ERR_PTR(-EAGAIN);
 }
 
 struct crypto_tfm *crypto_spawn_tfm(struct crypto_spawn *spawn, u32 type,
--- a/crypto/api.c
+++ b/crypto/api.c
@@ -345,13 +345,12 @@ static unsigned int crypto_ctxsize(struc
 	return len;
 }
 
-void crypto_shoot_alg(struct crypto_alg *alg)
+static void crypto_shoot_alg(struct crypto_alg *alg)
 {
 	down_write(&crypto_alg_sem);
 	alg->cra_flags |= CRYPTO_ALG_DYING;
 	up_write(&crypto_alg_sem);
 }
-EXPORT_SYMBOL_GPL(crypto_shoot_alg);
 
 struct crypto_tfm *__crypto_alloc_tfm(struct crypto_alg *alg, u32 type,
 				      u32 mask)
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -88,7 +88,6 @@ void crypto_alg_tested(const char *name,
 void crypto_remove_spawns(struct crypto_alg *alg, struct list_head *list,
 			  struct crypto_alg *nalg);
 void crypto_remove_final(struct list_head *list);
-void crypto_shoot_alg(struct crypto_alg *alg);
 struct crypto_tfm *__crypto_alloc_tfm(struct crypto_alg *alg, u32 type,
 				      u32 mask);
 void *crypto_create_tfm(struct crypto_alg *alg,