linux-kernel - Re: [PATCH] seqlock: serialize against writers

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]

Date:	Fri, 29 Aug 2008 12:41:33 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Gregory Haskins <gregory.haskins@...il.com>, mingo@...e.hu,
	rostedt@...dmis.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] seqlock: serialize against writers

Andi Kleen wrote:
> On Fri, Aug 29, 2008 at 12:29:42PM -0400, Gregory Haskins wrote:
>   
>> Andi Kleen wrote:
>>     
>>>> Im running it on a x86_64 box as we speak.  How can I tell if there is a
>>>> certain mode that is permitting this?
>>>>     
>>>>         
>>> If the boot up says you're running with PMtimer then it uses the fallback
>>> (usually happens on pre Fam10h AMD boxes). A typical Intel box
>>> would use the faster ring 3 only TSC path and then explode with your
>>> change I bet. 
>>>   
>>>       
>> Thinking about this some more, perhaps the issue is I am not hitting the
>> contended path in vsyscall?
>>     
>
> Yes it will be only contended when gettimeofday() races with the timer 
> interrupt.  You could try to run gettimeofday() in a loop and see how
> long it holds up.
>
> But anyways from the theory you should crash when it happens. 
> Writes to kernel data are not allowed in vsyscalls and your read_lock clearly 
> does a write.
>   

Oh I don't deny that it does.  The compiler neatly reminded me that it
could no longer be "const".  I just was ignorant of the userspace
requirement ;)

But we *do* have a serious problem here.  A non-preemptible seqlock_t
will do bad things if it preempts the writer, so we need some kind of
solution here, one way or the other.  So suggestions welcome :)  I
realize this is only an issue currently in PREEMPT_RT so perhaps most
will not care... but I do need to solve this at least for this branch.

I currently do not see a way to solve this problem that doesn't involve
some heavier involvement in the read path (read: const seqlock_t need
not apply).  Given that, we either need to address the const requirement
for userspace, alter userspace's usage, or find another way to address
the deadlock.  Any ideas?

-Greg

Download attachment "signature.asc" of type "application/pgp-signature" (258 bytes)