linux-kernel - [PATCH] futexs: fix infinite loop in get_futex

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Message-ID: <20090710231313.GA23572@us.ibm.com>
Date:	Fri, 10 Jul 2009 18:13:13 -0500
From:	Sonny Rao <sonnyrao@...ibm.com>
To:	tglx@...uxtronix.de
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	linux-mm@...r.kernel.org, sonnyrao@...ibm.com, stable@...nel.org,
	anton@...ba.org, rajamony@...ibm.com, speight@...ibm.com,
	mstephen@...ibm.com, grimm@...ibm.com, mikey@...abs.au.ibm.com
Subject: [PATCH] futexs: fix infinite loop in get_futex_key on huge page

get_futex_key() can infinitely loop if it is called on a virtual address
that is within a huge page but not aligned to the beginning of that
page.  The call to get_user_pages_fast will return the struct page for
a sub-page within the huge page and the check for page->mapping will
always fail.

The fix is to call compound_head on the page before checking that it's mapped.

Signed-off-by: Sonny Rao <sonnyrao@...ibm.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: stable@...nel.org

Index: linux-2.6.31-rc2/kernel/futex.c
===================================================================
--- linux-2.6.31-rc2.orig/kernel/futex.c	2009-07-10 17:45:46.181084475 -0500
+++ linux-2.6.31-rc2/kernel/futex.c	2009-07-10 17:46:47.345084062 -0500
@@ -247,6 +247,7 @@
 	if (err < 0)
 		return err;
 
+	page = compound_head(page);
 	lock_page(page);
 	if (!page->mapping) {
 		unlock_page(page);
--
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