[ubuntu-hardened] [kernel-hardening] Re: Add overflow protection to kref
dwindsor at gmail.com
Fri Feb 24 19:04:57 UTC 2012
On Fri, Feb 24, 2012 at 1:37 PM, Greg KH <gregkh at linuxfoundation.org> wrote:
> On Fri, Feb 24, 2012 at 12:58:35PM -0500, David Windsor wrote:
>> >> 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.
>> Signed-off-by: David Windsor <dwindsor at gmail.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;
>> - 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
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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