linux-kernel - [BUG] rmap: fix page_address_in_vma() to walk through anon_vma

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Date:	Thu, 22 Apr 2010 14:42:41 +0900
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	linux-mm <linux-mm@...ck.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: [BUG] rmap: fix page_address_in_vma() to walk through
 anon_vma_chain

I found a bug on page_address_in_vma() related to anon_vma_chain.

I wrote a patch, but according to a comment in include/linux/rmap.h,
I suspect this doesn't meet lock requirement of anon_vma_chain
(mmap_sem and page_table_lock, see below).

                           mmap_sem      page_table_lock
  mm/ksm.c:
    write_protect_page()   hold          not hold
    replace_page()         hold          not hold
  mm/memory-failure.c:
    add_to_kill()          not hold      hold
  mm/mempolicy.c:
    new_vma_page()         hold          not hold
  mm/swapfile.c:
    unuse_vma()            hold          not hold

Any comments?

Thanks,
Naoya Horiguchi
---
Subject: [BUG] rmap: fix page_address_in_vma() to walk through anon_vma_chain

page_address_in_vma() checks if a given page is associated with a given vma.
Currently it just compares vma->anon_vma and page_anon_vma(page).
But in 2.6.34, a vma can have multiple anon_vmas with anon_vma_chain,
so we have to check all anon_vmas in the "same_vma" chain.
Otherwise, when a page is shared by multiple processes,
some (page,vma) pairs can be misjudged as not-mapped.

Need Work: Meet lock requirement of anon_vma_chain.

Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Andi Kleen <andi@...stfloor.org>
---
 mm/rmap.c |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 526704e..2e7462b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -340,14 +340,22 @@ vma_address(struct page *page, struct vm_area_struct *vma)
 unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
 {
 	if (PageAnon(page)) {
-		if (vma->anon_vma != page_anon_vma(page))
-			return -EFAULT;
+		struct anon_vma_chain *avc;
+		/*
+		 * Walking same_vma list needs mmap_sem and page_table_lock.
+		 * Do users of this function meet it?
+		 */
+		list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
+			if (avc->anon_vma == page_anon_vma(page))
+				goto get_address;
+		return -EFAULT;
 	} else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
 		if (!vma->vm_file ||
 		    vma->vm_file->f_mapping != page->mapping)
 			return -EFAULT;
 	} else
 		return -EFAULT;
+get_address:
 	return vma_address(page, vma);
 }
 
-- 
1.7.0
--
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