linux-kernel - Re: Question about x86/mm/gup.c's use of disabled interrupts

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, 18 Mar 2009 23:40:24 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: Question about x86/mm/gup.c's use of disabled interrupts

Jeremy Fitzhardinge wrote:
>>> Disabling the interrupt will prevent the tlb flush IPI from coming 
>>> in and flushing this cpu's tlb, but I don't see how it will prevent 
>>> some other cpu from actually updating the pte in the pagetable, 
>>> which is what we're concerned about here.  
>>
>> The thread that cleared the pte holds the pte lock and is now waiting 
>> for the IPI.  The thread that wants to update the pte will wait for 
>> the pte lock, thus also waits on the IPI and gup_fast()'s 
>> local_irq_enable().  I think.
>
> But hasn't it already done the pte update at that point?
>
> (I think this conversation really is moot because the kernel never 
> does P->P pte updates any more; its always P->N->P.)

I thought you were concerned about cpu 0 doing a gup_fast(), cpu 1 doing 
P->N, and cpu 2 doing N->P.  In this case cpu 2 is waiting on the pte lock.

>>> Is this the only reason to disable interrupts?  
>>
>> Another comment says it also prevents pagetable teardown.
>
> We could take a reference to the mm to get the same effect, no?
>

Won't stop munmap().

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

--
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/