linux-kernel - Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

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:	Wed, 11 Feb 2015 16:38:58 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Dave Jones <davej@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Jason Wang <jasowang@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	xen-devel@...ts.xenproject.org, Rik van Riel <riel@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>
Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

On 02/10/2015 06:56 PM, Oleg Nesterov wrote:
> On 02/10, Raghavendra K T wrote:
>>
>> On 02/10/2015 06:23 AM, Linus Torvalds wrote:
>>
>>>           add_smp(&lock->tickets.head, TICKET_LOCK_INC);
>>>           if (READ_ONCE(lock->tickets.tail) & TICKET_SLOWPATH_FLAG) ..
>>>
>>> into something like
>>>
>>>           val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC << TICKET_SHIFT);
>>>           if (unlikely(val & TICKET_SLOWPATH_FLAG)) ...
>>>
>>> would be the right thing to do. Somebody should just check that I got
>>> that shift right, and that the tail is in the high bytes (head really
>>> needs to be high to work, if it's in the low byte(s) the xadd would
>>> overflow from head into tail which would be wrong).
>>
>> Unfortunately xadd could result in head overflow as tail is high.
>>
>> The other option was repeated cmpxchg which is bad I believe.
>> Any suggestions?
>
> Stupid question... what if we simply move SLOWPATH from .tail to .head?
> In this case arch_spin_unlock() could do xadd(tickets.head) and check
> the result

It is a good idea. Trying this now.

> In this case __ticket_check_and_clear_slowpath() really needs to cmpxchg
> the whole .head_tail. Plus obviously more boring changes. This needs a
> separate patch even _if_ this can work.

Correct, but apart from this, before doing xadd in unlock,
we would have to make sure lsb bit is cleared so that we can live with 1 
bit overflow to tail which is unused. now either or both of head,tail
lsb bit may be set after unlock.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/