Re: mm: NULL ptr deref in unlink_file_vma

[Kirill A. Shutemov]

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?

For now, I would propose to revert the commit and probably re-introduce it
after v3.19: