linux-kernel - Re: [kernel-hardening] Re: Add overflow protection to kref

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Date:	Fri, 24 Feb 2012 12:58:35 -0500
From:	David Windsor <dwindsor@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Roland Dreier <roland@...estorage.com>,
	Djalal Harouni <tixxdz@...ndz.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	kernel-hardening@...ts.openwall.com,
	Kees Cook <keescook@...omium.org>,
	Ubuntu security discussion <ubuntu-hardened@...ts.ubuntu.com>,
	linux-kernel@...r.kernel.org, pageexec@...email.hu,
	spender@...ecurity.net
Subject: Re: [kernel-hardening] Re: Add overflow protection to kref

<snip>

>> Greg, I'm not sure why you're opposed to adding this checking...
>> it's pretty clear that buggy error paths that forget to do a put are
>> pretty common and will continue to be common in new code, and
>> making them harder to exploit seems pretty sane to me.
>>
>> What's the downside?
>
> The downside is that there has not even been a patch sent for any of
> this.  Combine that with a lack of understanding about reference
> counting and atomic_t usages in the kernel, and the whole thing is ripe
> for misunderstanding and confusion.
>
> greg k-h

This approach to adding overflow protection to kref uses
atomic_add_unless to increment the refcounter only if it is not
already at INT_MAX.  This
leaks the internal representation of atomic_t, which is defined as an
int in linux/types.h, into kref.

If we can agree on an approach to adding overflow protection, if it is
indeed desired, we can then discuss adding a Kconfig option and/or a
sysctl for this protection.

Thanks,
David


Signed-off-by: David Windsor <dwindsor@...il.com>
---
 include/linux/kref.h |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 9c07dce..fc0756a 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -38,8 +38,12 @@ static inline void kref_init(struct kref *kref)
  */
 static inline void kref_get(struct kref *kref)
 {
+   int rc = 0;
    WARN_ON(!atomic_read(&kref->refcount));
-   atomic_inc(&kref->refcount);
+   smp_mb__before_atomic_inc();
+   rc = atomic_add_unless(&kref->refcount, 1, INT_MAX);
+   smp_mb__after_atomic_inc();
+   BUG_ON(!rc);
 }

 /**
-- 
1.7.5.4


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