linux-kernel - Re: [PATCH] mm: protect against concurrent vma expansion

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: <CANN689HQjbXEpWhv5KuaOt2NBEokiOguCXnsum2Bd994zkw6tA@mail.gmail.com>
Date:	Thu, 3 Jan 2013 16:50:20 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Simon Jeons <simon.jeons@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: protect against concurrent vma expansion

On Thu, Jan 3, 2013 at 4:40 PM, Simon Jeons <simon.jeons@...il.com> wrote:
> On Wed, 2012-12-19 at 19:01 -0800, Michel Lespinasse wrote:
>> Hi Simon,
>>
>> On Wed, Dec 19, 2012 at 5:56 PM, Simon Jeons <simon.jeons@...il.com> wrote:
>> > One question.
>> >
>> > I found that mainly callsite of expand_stack() is #PF, but it holds
>> > mmap_sem each time before call expand_stack(), how can hold a *shared*
>> > mmap_sem happen?
>>
>> the #PF handler calls down_read(&mm->mmap_sem) before calling expand_stack.
>>
>> I think I'm just confusing you with my terminology; shared lock ==
>> read lock == several readers might hold it at once (I'd say they share
>> it)
>
> Sorry for my late response.
>
> Since expand_stack() will modify vma, then why hold a read lock here?

Well, it'd be much nicer if we had a write lock, I think. But, we
didn't know when taking the lock that we'd end up having to expand
stacks.

What happens is that page faults don't generally modify vmas, so they
get a read lock (just to know what vma the fault is happening in) and
then fault in the page.

expand_stack() is the one exception to that - after getting the read
lock as usual, we notice that the fault is not in any vma right now,
but it's close enough to an expandable vma.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
--
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/