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Date:	Wed, 11 Feb 2015 14:22:24 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, Hugh Dickins <hughd@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: mm: NULL ptr deref in unlink_file_vma

On Tue, Feb 10, 2015 at 10:42:31PM +0400, Konstantin Khlebnikov wrote:
> On Mon, Dec 22, 2014 at 10:14 PM, Kirill A. Shutemov
> <kirill@...temov.name> wrote:
> > On Mon, Dec 22, 2014 at 01:05:13PM -0500, Sasha Levin wrote:
> >> On 12/22/2014 01:01 PM, Kirill A. Shutemov wrote:
> >> > On Mon, Dec 22, 2014 at 10:04:02AM -0500, Sasha Levin wrote:
> >> >> > Hi all,
> >> >> >
> >> >> > While fuzzing with trinity inside a KVM tools guest running the latest -next
> >> >> > kernel, I've stumbled on the following spew:
> >> >> >
> >> >> > [  432.376425] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
> >> >> > [  432.378876] IP: down_write (./arch/x86/include/asm/rwsem.h:105 ./arch/x86/include/asm/rwsem.h:121 kernel/locking/rwsem.c:71)
> >> > Looks like vma->vm_file->mapping is NULL. Somebody freed ->vm_file from
> >> > under us?
> >> >
> >> > I suspect Davidlohr's patchset on i_mmap_lock, but I cannot find any code
> >> > path which could lead to the crash.
> >>
> >> I've reported a different issue which that patchset: https://lkml.org/lkml/2014/12/9/741
> >>
> >> I guess it could be related?
> >
> > Maybe.
> >
> > Other thing:
> >
> >  unmap_mapping_range()
> >    i_mmap_lock_read(mapping);
> >    unmap_mapping_range_tree()
> >      unmap_mapping_range_vma()
> >        zap_page_range_single()
> >          unmap_single_vma()
> >            untrack_pfn()
> >              vma->vm_flags &= ~VM_PAT;
> >
> > It seems we modify ->vm_flags without mmap_sem taken, means we can corrupt
> > them.
> >
> > Sasha could you check if you hit untrack_pfn()?
> >
> > The problem probably was hidden by exclusive i_mmap_lock on
> > unmap_mapping_range(), but it's not exclusive anymore afrer Dave's
> > patchset.
> >
> > Konstantin, you've modified untrack_pfn() back in 2012 to change
> > ->vm_flags. Any coments?
> 
> Hmm. I don't really understand how
> unmap_mapping_range() could be used for VM_PFNMAP mappings
> except unmap() or exit_mmap() where mm is locked anyway.
> Somebody truncates memory mapped device and unmaps mapped PFNs?

Hm. Probably not. But it's not obvious to me what would stop this.
Should we at least have assert on mmap_sem locked in untrack_pfn()?

> If it's a problem then I think VM_PAT could be tuned into hint which
> means PAT tracking was here and we pat should check internal structure
> for details and take actions if pat tracking is still here. As I see
> pat tracking probably also have problems if somebody unmaps that vma
> partially.

IIUC, we only mark a vma with VM_PAT if whole vma is subject for
remap_pfn_range(). I don't see a point in cleaning VM_PAT -- just let it
die with vma. Or do I miss something?

-- 
 Kirill A. Shutemov
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