linux-kernel - [PATCH 3.2 28/67] x86/mm/pat: Avoid truncation when converting cpa->numpages to address

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Message-ID: <lsq.1456263723.307974614@decadent.org.uk>
Date:	Tue, 23 Feb 2016 21:42:03 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Thomas Gleixner" <tglx@...utronix.de>,
	"Matt Fleming" <matt@...eblueprint.co.uk>,
	"Borislav Petkov" <bp@...en8.de>,
	"Sai Praneeth Prakhya" <sai.praneeth.prakhya@...el.com>
Subject: [PATCH 3.2 28/67] x86/mm/pat: Avoid truncation when converting
 cpa->numpages to address

3.2.78-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Matt Fleming <matt@...eblueprint.co.uk>

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@...il.com>
Acked-by: Borislav Petkov <bp@...en8.de>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Signed-off-by: Matt Fleming <matt@...eblueprint.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@...utronix.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/mm/pageattr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -32,7 +32,7 @@ struct cpa_data {
 	unsigned long	*vaddr;
 	pgprot_t	mask_set;
 	pgprot_t	mask_clr;
-	int		numpages;
+	unsigned long	numpages;
 	int		flags;
 	unsigned long	pfn;
 	unsigned	force_split : 1;
@@ -820,7 +820,7 @@ static int __change_page_attr_set_clr(st
 		 * CPA operation. Either a large page has been
 		 * preserved or a single page update happened.
 		 */
-		BUG_ON(cpa->numpages > numpages);
+		BUG_ON(cpa->numpages > numpages || !cpa->numpages);
 		numpages -= cpa->numpages;
 		if (cpa->flags & (CPA_PAGES_ARRAY | CPA_ARRAY))
 			cpa->curpage++;