Re: mm: NULL ptr deref in unlink_file_vma

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 2014-12-22 at 21:14 +0200, Kirill A. Shutemov wrote:
> 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.

yep. Although one thing that wouldn't match this would be the mlock'd
bad page when freeing in both of Sasha's previous reports, as we would
need to have VM_PFNMAP when calling untrack_pfn().

> 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:
> 
> From 14392c69fcfeeda34eb9f75d983dad32698cdd5c Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Date: Mon, 22 Dec 2014 21:01:54 +0200
> Subject: [PATCH] Revert "mm/memory.c: share the i_mmap_rwsem"
> 
> This reverts commit c8475d144abb1e62958cc5ec281d2a9e161c1946.
> 
> There are several[1][2] of bug reports which points to this commit as potential
> cause[3].
> 
> Let's revert it until we figure out what's going on.
> 
> [1] https://lkml.org/lkml/2014/11/14/342
> [2] https://lkml.org/lkml/2014/12/22/213
> [3] https://lkml.org/lkml/2014/12/9/741
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Cc: Davidlohr Bueso <dave@stgolabs.net>

I certainly have no problem with this. Furthermore we snuck this one in
kinda last minute, so:

Acked-by: Davidlohr Bueso <dave@stgolabs.net>

Thanks,
Davidlohr

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