Re: mm: NULL ptr deref in unlink_file_vma

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 2014-12-22 at 20:04 +0200, Kirill A. Shutemov wrote:
> [ fixed Davidlohr's address. ]
> 
> On Mon, Dec 22, 2014 at 08:01:02PM +0200, 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.

Sasha, does this still occur if you revert c8475d144abb?

> > I've noticed one strange code path, which probably is not related to the
> > issue:
> > 
> > unmap_mapping_range()
> >   i_mmap_lock_read(mapping);
> >   unmap_mapping_range_tree()
> >     unmap_mapping_range_vma()
> >       zap_page_range_single()
> >         unmap_single_vma()
> > 	  if (unlikely(is_vm_hugetlb_page(vma))) {
> > 	    i_mmap_lock_write(vma->vm_file->f_mapping);

Right, this is would be completely bogus. But the deadlock cannot happen
in reality as hugetlb uses its own handlers and thus never calls
unmap_mapping_range.

Thanks,
Davidlohr

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