On 07/09/2013 19:32, Herbert Xu wrote:
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;
}
I tried this patch, but I still see an apparent module lookup/load race
if code on several CPUs calls crypto_alloc_aead at the same time, and an
external module such as aes needs to be loaded.
Seeing this in the log: "request_module: runaway loop modprobe gcm(aes)"
Shouldn't module lookup/load be bracketed by some sort of lock to
prevent this?
James
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