linux-kernel - [PATCH 071/173] mm: fix possible cause of a page

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Message-Id: <20110425200236.079177121@pcw.home.local>
Date:	Mon, 25 Apr 2011 22:03:43 +0200
From:	Willy Tarreau <w@....eu>
To:	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org
Cc:	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kerin Millar <kerframil@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: [PATCH 071/173] mm: fix possible cause of a page_mapped BUG

2.6.27.59-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Hugh Dickins <hughd@...gle.com>

commit a3e8cc643d22d2c8ed36b9be7d9c9ca21efcf7f7 upstream.

Robert Swiecki reported a BUG_ON(page_mapped) from a fuzzer, punching
a hole with madvise(,, MADV_REMOVE).  That path is under mutex, and
cannot be explained by lack of serialization in unmap_mapping_range().

Reviewing the code, I found one place where vm_truncate_count handling
should have been updated, when I switched at the last minute from one
way of managing the restart_addr to another: mremap move changes the
virtual addresses, so it ought to adjust the restart_addr.

But rather than exporting the notion of restart_addr from memory.c, or
converting to restart_pgoff throughout, simply reset vm_truncate_count
to 0 to force a rescan if mremap move races with preempted truncation.

We have no confirmation that this fixes Robert's BUG,
but it is a fix that's worth making anyway.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kerin Millar <kerframil@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 mm/mremap.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -92,9 +92,7 @@ static void move_ptes(struct vm_area_str
 		 */
 		mapping = vma->vm_file->f_mapping;
 		spin_lock(&mapping->i_mmap_lock);
-		if (new_vma->vm_truncate_count &&
-		    new_vma->vm_truncate_count != vma->vm_truncate_count)
-			new_vma->vm_truncate_count = 0;
+		new_vma->vm_truncate_count = 0;
 	}

 	/*

--
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