There are two problems in crypto_spawn_alg. First of all it may
return spawn->alg even if spawn->dead is set. This results in a
double-free as detected by syzbot.
Secondly the setting of the DYING flag is racy because we hold
the read-lock instead of the write-lock. We should instead call
crypto_shoot_alg in a safe manner by gaining a refcount, dropping
the lock, and then releasing the refcount.
This patch fixes both problems.
Reported-by: [email protected]
Fixes: 4f87ee118d16 ("crypto: api - Do not zap spawn->alg")
Fixes: 73669cc55646 ("crypto: api - Fix race condition in...")
Cc: <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>
diff --git a/crypto/algapi.c b/crypto/algapi.c
index 69605e21af92..f8b4dc161c02 100644
--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -716,17 +716,27 @@ EXPORT_SYMBOL_GPL(crypto_drop_spawn);
static struct crypto_alg *crypto_spawn_alg(struct crypto_spawn *spawn)
{
- struct crypto_alg *alg;
+ struct crypto_alg *alg = ERR_PTR(-EAGAIN);
+ struct crypto_alg *target;
+ bool shoot = false;
down_read(&crypto_alg_sem);
- alg = spawn->alg;
- if (!spawn->dead && !crypto_mod_get(alg)) {
- alg->cra_flags |= CRYPTO_ALG_DYING;
- alg = NULL;
+ if (!spawn->dead) {
+ alg = spawn->alg;
+ if (!crypto_mod_get(alg)) {
+ target = crypto_alg_get(alg);
+ shoot = true;
+ alg = ERR_PTR(-EAGAIN);
+ }
}
up_read(&crypto_alg_sem);
- return alg ?: ERR_PTR(-EAGAIN);
+ if (shoot) {
+ crypto_shoot_alg(target);
+ crypto_alg_put(target);
+ }
+
+ return alg;
}
struct crypto_tfm *crypto_spawn_tfm(struct crypto_spawn *spawn, u32 type,
diff --git a/crypto/api.c b/crypto/api.c
index 7d71a9b10e5f..edcf690800d4 100644
--- a/crypto/api.c
+++ b/crypto/api.c
@@ -333,12 +333,13 @@ static unsigned int crypto_ctxsize(struct crypto_alg *alg, u32 type, u32 mask)
return len;
}
-static void crypto_shoot_alg(struct crypto_alg *alg)
+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)
diff --git a/crypto/internal.h b/crypto/internal.h
index d5ebc60c5143..ff06a3bd1ca1 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -65,6 +65,7 @@ void crypto_alg_tested(const char *name, int err);
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,
--
Email: Herbert Xu <[email protected]>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
> Secondly the setting of the DYING flag is racy because we hold
> the read-lock instead of the write-lock. We should instead call
Would an imperative wording be preferred for this change description?
> This patch fixes both problems.
Will another wording alternative become relevant?
Are the provided tags sufficient for such information?
Regards,
Markus
On Fri, Apr 10, 2020 at 04:09:42PM +1000, Herbert Xu wrote:
> There are two problems in crypto_spawn_alg. First of all it may
> return spawn->alg even if spawn->dead is set. This results in a
> double-free as detected by syzbot.
>
> Secondly the setting of the DYING flag is racy because we hold
> the read-lock instead of the write-lock. We should instead call
> crypto_shoot_alg in a safe manner by gaining a refcount, dropping
> the lock, and then releasing the refcount.
>
> This patch fixes both problems.
>
> Reported-by: [email protected]
> Fixes: 4f87ee118d16 ("crypto: api - Do not zap spawn->alg")
> Fixes: 73669cc55646 ("crypto: api - Fix race condition in...")
> Cc: <[email protected]>
> Signed-off-by: Herbert Xu <[email protected]>
>
> diff --git a/crypto/algapi.c b/crypto/algapi.c
> index 69605e21af92..f8b4dc161c02 100644
> --- a/crypto/algapi.c
> +++ b/crypto/algapi.c
> @@ -716,17 +716,27 @@ EXPORT_SYMBOL_GPL(crypto_drop_spawn);
>
> static struct crypto_alg *crypto_spawn_alg(struct crypto_spawn *spawn)
> {
> - struct crypto_alg *alg;
> + struct crypto_alg *alg = ERR_PTR(-EAGAIN);
> + struct crypto_alg *target;
> + bool shoot = false;
>
> down_read(&crypto_alg_sem);
> - alg = spawn->alg;
> - if (!spawn->dead && !crypto_mod_get(alg)) {
> - alg->cra_flags |= CRYPTO_ALG_DYING;
> - alg = NULL;
> + if (!spawn->dead) {
> + alg = spawn->alg;
> + if (!crypto_mod_get(alg)) {
> + target = crypto_alg_get(alg);
> + shoot = true;
> + alg = ERR_PTR(-EAGAIN);
> + }
> }
> up_read(&crypto_alg_sem);
>
> - return alg ?: ERR_PTR(-EAGAIN);
> + if (shoot) {
> + crypto_shoot_alg(target);
> + crypto_alg_put(target);
> + }
> +
> + return alg;
> }
Wouldn't it be a bit simpler to set 'target = NULL', remove 'shoot',
and use 'if (target)' instead of 'if (shoot)'?
- Eric
On Wed, Apr 15, 2020 at 07:30:01PM -0700, Eric Biggers wrote:
>
> I'm not sure what you mean here. crypto_alg_get() is:
>
> static inline struct crypto_alg *crypto_alg_get(struct crypto_alg *alg)
> {
> refcount_inc(&alg->cra_refcnt);
> return alg;
> }
>
> So given:
>
> target = crypto_alg_get(alg);
>
> Both alg and target have to be non-NULL.
Yes I know that we know that it can't be NULL, but gcc 8.3 doesn't.
Cheers,
--
Email: Herbert Xu <[email protected]>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt