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

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Message-ID: <CAEXv5_jEW6EA6nOd6pMJbh6PTd=+piHqzNA8Fb+gt2VGroba0A@mail.gmail.com>
Date:	Fri, 24 Feb 2012 14:04:57 -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

On Fri, Feb 24, 2012 at 1:37 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Fri, Feb 24, 2012 at 12:58:35PM -0500, David Windsor wrote:
>> <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);
>
> So you are guaranteeing to crash a machine here if this fails?  And you
> were trying to say this is a "security" based fix?
>


Suggestions on recovering from an overflow here?

The salient possibilities are:

1. Do nothing, as was the case before this patch.
This leads to undefined behavior, but typically, a wrap happens.  This
leads to use-after-free bugs.

2. Detect the overflow before it happens, don't increment the counter,
issue a warning but no BUG.
This could also lead to some "undefined" behavior in subsystems.  I
can't imagine many users of kref have considered the situation of
kref_get essentially being a no-op, which is what would happen in this
situation.

3. Detect the overflow before it happens, don't increment the counter,
issue a BUG.
This is what the proposed patch does.  Suggestions for recovering
gracefully from this overflow are welcome.

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