linux-kernel - Re: tip -ENOBOOT - bisected to locking/refcounts, x86/asm: Implement fast refcount overflow protection

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAGXu5jKYPp5D+YxtqrJXsi48UUbjfeREngsys-dU1eP4RnnL+w@mail.gmail.com>
Date:   Wed, 30 Aug 2017 21:10:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mike Galbraith <efault@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: tip -ENOBOOT - bisected to locking/refcounts, x86/asm: Implement
 fast refcount overflow protection

On Wed, Aug 30, 2017 at 9:01 PM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Aug 30, 2017 at 8:12 PM, Mike Galbraith <efault@....de> wrote:
>> On Wed, 2017-08-30 at 19:27 -0700, Kees Cook wrote:
>>
>>> Interesting! Can you try with 633547973ffc3 ("net: convert
>>> sk_buff.users from atomic_t to refcount_t") reverted? I'll see if
>>> running haveged will help me trigger this on my system...
>>
>> With that (plus 230cd1279d001 fix to it) reverted, vbox boots.
>
> Wonderful! Thank you so much for helping track this down.
>
> So, it seems that sk_buff.users will need some more special attention
> before we can convert it to refcount.
>
> x86-refcount will saturate with refcount_dec_and_test() if the result
> is negative. But that would mean at least starting at 0. FULL should
> have WARNed in this case, so I remain slightly confused why it was
> missed by FULL.

Actually, if this is a race condition it's possible that FULL is slow
enough to miss it...

I bet something briefly takes the refcount negative, and with
unchecked atomics, it come back up positive again during the race.
FULL may miss the race, and x86-refcount will catch it and saturate...

-Kees

-- 
Kees Cook
Pixel Security