linux-kernel - Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all

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: <1270140592.1598.153.camel@laptop>
Date:	Thu, 01 Apr 2010 18:49:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	Kent Overstreet <kent.overstreet@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in
 mmu_take_all_locks()

On Thu, 2010-04-01 at 18:45 +0200, Peter Zijlstra wrote:
> On Thu, 2010-04-01 at 18:18 +0200, Andrea Arcangeli wrote:
> > On Thu, Apr 01, 2010 at 06:12:34PM +0200, Peter Zijlstra wrote:
> > > One thing we can do there is to mutex_trylock() if we get the lock, see
> > > if we've got the right object, if the trylock fails we can do the
> > > refcount thing and sleep. That would allow the fast-path to remain a
> > > single atomic.
> > 
> > But then how do you know which anon_vma_unlink has to decrease the
> > refcount and which not? That info should need to be stored in the
> > kernel stack, can't be stored in the vma. I guess it's feasible but
> > passing that info around sounds more tricky than the trylock itself
> > (adding params to those functions with int &refcount). 
> 
> I was thinking of something like:
> 
> struct anon_vma *page_lock_anon_vma(struct page *page)
> {
> 	struct anon_vma *anon_vma = NULL;
> 	unsigned long anon_mapping;
> 
> 	rcu_read_lock();
> 	anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
>         if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)              
>                 goto out;                                                          
>         if (!page_mapped(page))                                                    
>                 goto out;                                                          
> 
>         anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);         
>         if (!mutex_trylock(&anon_vma->lock)) {
> 		if (atomic_inc_unless_zero(&anon_vma->ref)) {
> 			rcu_read_unlock();
> 			mutex_lock(&anon_vma->lock);
> 			atomic_dec(&anon_vma->ref); /* ensure the lock pins it */
> 		} else
> 			anon_vma = NULL;
> 	}
> 	rcu_read_unlock();
> 	
>         return anon_vma;
> }
> 
> void page_unlock_anon_vma(struct anon_vma *anon_vma)
> {
> 	mutex_unlock(&anon_vma->lock);	
> }
> 
> Then anybody reaching ref==0 would only need to sync against the lock
> before freeing.

Ah, there is a race where the dec after lock makes it 0, we could catch
that by making it -1 and free in unlock_anon_vma().

--
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/