linux-kernel - Re: sched: 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]

Message-ID: <54D3DA75.70402@oracle.com>
Date:	Thu, 05 Feb 2015 16:02:45 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Davidlohr Bueso <dave@...olabs.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Waiman Long <Waiman.Long@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: sched: memory corruption on completing completions

On 02/05/2015 03:59 PM, Davidlohr Bueso wrote:
> On Wed, 2015-02-04 at 16:16 -0800, Linus Torvalds wrote:
>> And looking at the arch version, I think the paravirtualized code is crap.
>>
>> It does:
>>
>>                 prev = *lock;
>>                 add_smp(&lock->tickets.head, TICKET_LOCK_INC);
>>
>>                 /* add_smp() is a full mb() */
>>
>>                 if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
>>                         __ticket_unlock_slowpath(lock, prev);
>>
>>
>> which is *exactly* the kind of things you cannot do with spinlocks,
>> because after you've done the "add_smp()" and released the spinlock
>> for the fast-path, you can't access the spinlock any more.  Exactly
>> because a fast-path lock migth come in, and release the whole data
>> structure.
>>
>> As usual, the paravirt code is a horribly buggy heap of crud. Film at 11.
> 
> Per http://lwn.net/Articles/495597/ which clearly describes the intent
> of the slowpath unlocking. Cc'ing Raghavendra.

Interestingly enough, according to that article this behaviour seems to be
"by design":

"""
This version of the patch uses a locked add to do this, followed by a test
to see if the slowflag is set.  The lock prefix acts as a full memory barrier,
so we can be sure that other CPUs will have seen the unlock before we read
the flag
"""

Thanks,
Sasha

--
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/