linux-kernel - Re: race condition in crypto larval handling

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Date:	Sun, 8 Sep 2013 11:32:11 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Kees Cook <keescook@...omium.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org, Tyler Hicks <tyhicks@...onical.com>
Subject: Re: race condition in crypto larval handling

On Fri, Sep 06, 2013 at 04:20:50PM -0700, Kees Cook wrote:
>
> In the two-thread situation, the first thread gets a larval with
> refcnt 2 via crypto_larval_add. (Why 2?) The next thread finds the
> larval via crypto_larval_add's call to __crypto_alg_lookup() and sees
> the ref bump to 3. While exiting crypto_alg_mod_lookup, each thread
> decrements the ref count twice.
> 
> It seems to me like either each call to crypto_larval_lookup() should
> result in a refcount bumped by two, or crypto_alg_mod_lookup() should
> decrement only once, and the initial refcount should be 1 not 2 from
> crypto_larval_add. But it's not clear to me which is sensible here.
> 
> What's the right solution here?

First of all thanks a lot for tracking this problem down! It's
been bothering me for months but I was unable to find a good
reproducer.

So now that you've identified the problem, the solution is easy.
crypto_larval_lookup should only ever return a larval if it created
one.  Any larval created earlier must be waited on first before we
return.

So does this patch fix the crash for you?

diff --git a/crypto/api.c b/crypto/api.c
index 320ea4d..a2b39c5 100644
--- a/crypto/api.c
+++ b/crypto/api.c
@@ -34,6 +34,8 @@ EXPORT_SYMBOL_GPL(crypto_alg_sem);
 BLOCKING_NOTIFIER_HEAD(crypto_chain);
 EXPORT_SYMBOL_GPL(crypto_chain);
 
+static struct crypto_alg *crypto_larval_wait(struct crypto_alg *alg);
+
 struct crypto_alg *crypto_mod_get(struct crypto_alg *alg)
 {
 	return try_module_get(alg->cra_module) ? crypto_alg_get(alg) : NULL;
@@ -144,8 +146,11 @@ static struct crypto_alg *crypto_larval_add(const char *name, u32 type,
 	}
 	up_write(&crypto_alg_sem);
 
-	if (alg != &larval->alg)
+	if (alg != &larval->alg) {
 		kfree(larval);
+		if (crypto_is_larval(alg))
+			alg = crypto_larval_wait(alg);
+	}
 
 	return alg;
 }

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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