[3.16.y-ckt stable] Patch "arm64: __clear_user: handle exceptions on strb" has been added to staging queue

[Luis Henriques]

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

    arm64: __clear_user: handle exceptions on strb

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt2.

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

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From ebaf3f3027f183fdd6db923f7fd9845cb2052388 Mon Sep 17 00:00:00 2001
From: Kyle McMartin <kyle at redhat.com>
Date: Wed, 12 Nov 2014 21:07:44 +0000
Subject: arm64: __clear_user: handle exceptions on strb
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit 97fc15436b36ee3956efad83e22a557991f7d19d upstream.

ARM64 currently doesn't fix up faults on the single-byte (strb) case of
__clear_user... which means that we can cause a nasty kernel panic as an
ordinary user with any multiple PAGE_SIZE+1 read from /dev/zero.
i.e.: dd if=/dev/zero of=foo ibs=1 count=1 (or ibs=65537, etc.)

This is a pretty obscure bug in the general case since we'll only
__do_kernel_fault (since there's no extable entry for pc) if the
mmap_sem is contended. However, with CONFIG_DEBUG_VM enabled, we'll
always fault.

if (!down_read_trylock(&mm->mmap_sem)) {
	if (!user_mode(regs) && !search_exception_tables(regs->pc))
		goto no_context;
retry:
	down_read(&mm->mmap_sem);
} else {
	/*
	 * The above down_read_trylock() might have succeeded in
	 * which
	 * case, we'll have missed the might_sleep() from
	 * down_read().
	 */
	might_sleep();
	if (!user_mode(regs) && !search_exception_tables(regs->pc))
		goto no_context;
}

Fix that by adding an extable entry for the strb instruction, since it
touches user memory, similar to the other stores in __clear_user.

Signed-off-by: Kyle McMartin <kyle at redhat.com>
Reported-by: Miloš Prchlík <mprchlik at redhat.com>
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/arm64/lib/clear_user.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/lib/clear_user.S b/arch/arm64/lib/clear_user.S
index 6e0ed93d51fe..c17967fdf5f6 100644
--- a/arch/arm64/lib/clear_user.S
+++ b/arch/arm64/lib/clear_user.S
@@ -46,7 +46,7 @@ USER(9f, strh	wzr, [x0], #2	)
 	sub	x1, x1, #2
 4:	adds	x1, x1, #1
 	b.mi	5f
-	strb	wzr, [x0]
+USER(9f, strb	wzr, [x0]	)
 5:	mov	x0, #0
 	ret
 ENDPROC(__clear_user)
--
2.1.0