[3.19.y-ckt,stable] Patch "x86/mm/pat: Avoid truncation when converting cpa->numpages to address" has been added to the 3.19.y-ckt tree

This is a note to let you know that I have just added a patch titled

    x86/mm/pat: Avoid truncation when converting cpa->numpages to address

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt15.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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From 9c9baf90abb611f815034448aa247d112868dc15 Mon Sep 17 00:00:00 2001
From: Matt Fleming <matt@codeblueprint.co.uk>
Date: Fri, 29 Jan 2016 11:36:10 +0000
Subject: x86/mm/pat: Avoid truncation when converting cpa->numpages to address
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
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commit 742563777e8da62197d6cb4b99f4027f59454735 upstream.

There are a couple of nasty truncation bugs lurking in the pageattr
code that can be triggered when mapping EFI regions, e.g. when we pass
a cpa->pgd pointer. Because cpa->numpages is a 32-bit value, shifting
left by PAGE_SHIFT will truncate the resultant address to 32-bits.

Viorel-Cătălin managed to trigger this bug on his Dell machine that
provides a ~5GB EFI region which requires 1236992 pages to be mapped.
When calling populate_pud() the end of the region gets calculated
incorrectly in the following buggy expression,

  end = start + (cpa->numpages << PAGE_SHIFT);

And only 188416 pages are mapped. Next, populate_pud() gets invoked
for a second time because of the loop in __change_page_attr_set_clr(),
only this time no pages get mapped because shifting the remaining
number of pages (1048576) by PAGE_SHIFT is zero. At which point the
loop in __change_page_attr_set_clr() spins forever because we fail to
map progress.

Hitting this bug depends very much on the virtual address we pick to
map the large region at and how many pages we map on the initial run
through the loop. This explains why this issue was only recently hit
with the introduction of commit

  a5caa209ba9c ("x86/efi: Fix boot crash by mapping EFI memmap
   entries bottom-up at runtime, instead of top-down")

It's interesting to note that safe uses of cpa->numpages do exist in
the pageattr code. If instead of shifting ->numpages we multiply by
PAGE_SIZE, no truncation occurs because PAGE_SIZE is a UL value, and
so the result is unsigned long.

To avoid surprises when users try to convert very large cpa->numpages
values to addresses, change the data type from 'int' to 'unsigned
long', thereby making it suitable for shifting by PAGE_SHIFT without
any type casting.

The alternative would be to make liberal use of casting, but that is
far more likely to cause problems in the future when someone adds more
code and fails to cast properly; this bug was difficult enough to
track down in the first place.

Reported-and-tested-by: Viorel-Cătălin Răpițeanu <rapiteanu.catalin@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=110131
Link: http://lkml.kernel.org/r/1454067370-10374-1-git-send-email-matt@codeblueprint.co.uk
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
 arch/x86/mm/pageattr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--
1.9.1