Re: mm: NULL ptr deref in unlink_file_vma

[Konstantin Khlebnikov]

On Wed, Feb 11, 2015 at 3:22 PM, Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxx> 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()?

exit_mmap() runs without mmap_sem thus this should be something like thus:
WARN_ON_ONCE(atomic_read(&mm->mm_users) && !rwsem_is_locked(&mm->mmap_sem));

Clearing VM_MAYSHARE in __unmap_hugepage_range_final() has the same problem,
it's called from unmap_single_vma() as well as untrack_pfn().

Also clear_soft_dirty_pmd() clears VM_SOFTDIRTY under mmap_sem  locked for read.

>
>> 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?

Yep, for now track/untrack works only for whole vma and cannot handle
vma split or partial munmap.

>
> --
>  Kirill A. Shutemov
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