linux-kernel - Re: mm: shmem: NULL ptr deref in shmem

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Date:	Tue, 13 May 2014 23:24:58 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: mm: shmem: NULL ptr deref in shmem_fault

On 05/13/2014 06:20 PM, Hugh Dickins wrote:
> I haven't delved into the perf_even_mmap d_path (fs/dcache.c:2947) one,
> but the Sys_mremap one on file->f_op->f_unmapped_area sounds like what
> we have here: struct file has been freed.
> 
> I believe Al is innocent: I point a quivering finger at... Kirill.
> 
> Just guessing, but we know how fond trinity is of remap_file_pages(),
> and comparing old and new emulations shows that interesting
> 
> 	struct file *file = get_file(vma->vm_file);
>         addr = mmap_region(...);
> 	fput(file);
> 
> in mm/fremap.c's old emulation, but no get_file() and fput() around 
> the do_mmap_pgoff() in mm/mmap.c's new emulation.
> 
> Before it puts in the new, do_mmap_pgoff() might unmap the last reference
> to vma->vm_file, so emulation needs to take its own reference.  I'm not
> sure how that plays out nowadays with Al's deferred fput, but it does
> look suspicious to me.

I've tested it by reverting the remap_file_pages() patch, and the problem
seems to have disappeared.

Then, I've added it back again, wrapping the do_mmap_pgoff() call with
get_file() and fput(), and the problem is still gone.

Seems like that was the issue all along. I'll send a patch...


Thanks,
Sasha
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