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Date: Tue, 25 Jul 2017 12:11:08 -0700
From: Kees Cook <keescook@...omium.org>
To: Li Kun <hw.likun@...wei.com>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, Christoph Hellwig <hch@...radead.org>, 
	"Eric W. Biederman" <ebiederm@...ssion.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Jann Horn <jannh@...gle.com>, Eric Biggers <ebiggers3@...il.com>, 
	Elena Reshetova <elena.reshetova@...el.com>, Hans Liljestrand <ishkamiel@...il.com>, 
	Greg KH <gregkh@...uxfoundation.org>, Alexey Dobriyan <adobriyan@...il.com>, 
	"Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com, Davidlohr Bueso <dave@...olabs.net>, 
	Manfred Spraul <manfred@...orfullife.com>, "axboe@...nel.dk" <axboe@...nel.dk>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, "x86@...nel.org" <x86@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, "David S. Miller" <davem@...emloft.net>, Rik van Riel <riel@...hat.com>, 
	LKML <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v8 3/3] x86/refcount: Implement fast
 refcount overflow protection

On Tue, Jul 25, 2017 at 5:03 AM, Li Kun <hw.likun@...wei.com> wrote:
> Hi Kees,
>
>
> on 2017/7/25 2:35, Kees Cook wrote:
>>
>> +static __always_inline __must_check
>> +int __refcount_add_unless(refcount_t *r, int a, int u)
>> +{
>> +       int c, new;
>> +
>> +       c = atomic_read(&(r->refs));
>> +       do {
>> +               if (unlikely(c == u))
>> +                       break;
>> +
>> +               asm volatile("addl %2,%0\n\t"
>> +                       REFCOUNT_CHECK_LT_ZERO
>> +                       : "=r" (new)
>> +                       : "0" (c), "ir" (a),
>> +                         [counter] "m" (r->refs.counter)
>> +                       : "cc", "cx");
>> +
>> +       } while (!atomic_try_cmpxchg(&(r->refs), &c, new));
>> +
>> +       return c;
>> +}
>
> here when the result LT_ZERO, you will saturate the r->refs.counter and make
> the
>
> atomic_try_cmpxchg(&(r->refs), &c, new) bound to fail first time.
>
> maybe we can just saturate the value of variable "new" ?

Oh, good catch! Thanks. Actually, it's even worse than that: we'll
exit this function without the refcount being correctly saturated. The
final result will be INT_MIN / 2 + a. It's not terrible, but I should
have noticed this in testing. (There was a gap in my testing for the
_not_zero() overflows, which I've fixed now...)

I'll figure this out or revert to the logic I was using in v6...

-Kees

-- 
Kees Cook
Pixel Security

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