Patch "arm64: swp emulation: bound LL/SC retries before rescheduling" has been added to the 4.8-stable tree

Patch "arm64: swp emulation: bound LL/SC retries before rescheduling" has been added to the 4.8-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    arm64: swp emulation: bound LL/SC retries before rescheduling

to the 4.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     arm64-swp-emulation-bound-ll-sc-retries-before-rescheduling.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 1c5b51dfb7b4564008e0cadec5381a69e88b0d21 Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@arm.com>
Date: Mon, 4 Jul 2016 16:59:43 +0100
Subject: arm64: swp emulation: bound LL/SC retries before rescheduling

From: Will Deacon <will.deacon@arm.com>

commit 1c5b51dfb7b4564008e0cadec5381a69e88b0d21 upstream.

If a CPU does not implement a global monitor for certain memory types,
then userspace can attempt a kernel DoS by issuing SWP instructions
targetting the problematic memory (for example, a framebuffer mapped
with non-cacheable attributes).

The SWP emulation code protects against these sorts of attacks by
checking for pending signals and potentially rescheduling when the STXR
instruction fails during the emulation. Whilst this is good for avoiding
livelock, it harms emulation of legitimate SWP instructions on CPUs
where forward progress is not guaranteed if there are memory accesses to
the same reservation granule (up to 2k) between the failing STXR and
the retry of the LDXR.

This patch solves the problem by retrying the STXR a bounded number of
times (4) before breaking out of the LL/SC loop and looking for
something else to do.

Fixes: bd35a4adc413 ("arm64: Port SWP/SWPB emulation support from arm")
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/arm64/kernel/armv8_deprecated.c |   36 +++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

--- a/arch/arm64/kernel/armv8_deprecated.c
+++ b/arch/arm64/kernel/armv8_deprecated.c
@@ -280,35 +280,43 @@ static void __init register_insn_emulati
 /*
  * Error-checking SWP macros implemented using ldxr{b}/stxr{b}
  */
-#define __user_swpX_asm(data, addr, res, temp, B)		\
+
+/* Arbitrary constant to ensure forward-progress of the LL/SC loop */
+#define __SWP_LL_SC_LOOPS	4
+
+#define __user_swpX_asm(data, addr, res, temp, temp2, B)	\
 	__asm__ __volatile__(					\
+	"	mov		%w3, %w7\n"			\
 	ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN,	\
 		    CONFIG_ARM64_PAN)				\
-	"0:	ldxr"B"		%w2, [%3]\n"			\
-	"1:	stxr"B"		%w0, %w1, [%3]\n"		\
+	"0:	ldxr"B"		%w2, [%4]\n"			\
+	"1:	stxr"B"		%w0, %w1, [%4]\n"		\
 	"	cbz		%w0, 2f\n"			\
-	"	mov		%w0, %w4\n"			\
+	"	sub		%w3, %w3, #1\n"			\
+	"	cbnz		%w3, 0b\n"			\
+	"	mov		%w0, %w5\n"			\
 	"	b		3f\n"				\
 	"2:\n"							\
 	"	mov		%w1, %w2\n"			\
 	"3:\n"							\
 	"	.pushsection	 .fixup,\"ax\"\n"		\
 	"	.align		2\n"				\
-	"4:	mov		%w0, %w5\n"			\
+	"4:	mov		%w0, %w6\n"			\
 	"	b		3b\n"				\
 	"	.popsection"					\
 	_ASM_EXTABLE(0b, 4b)					\
 	_ASM_EXTABLE(1b, 4b)					\
 	ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,	\
 		CONFIG_ARM64_PAN)				\
-	: "=&r" (res), "+r" (data), "=&r" (temp)		\
-	: "r" (addr), "i" (-EAGAIN), "i" (-EFAULT)		\
+	: "=&r" (res), "+r" (data), "=&r" (temp), "=&r" (temp2)	\
+	: "r" (addr), "i" (-EAGAIN), "i" (-EFAULT),		\
+	  "i" (__SWP_LL_SC_LOOPS)				\
 	: "memory")
 
-#define __user_swp_asm(data, addr, res, temp) \
-	__user_swpX_asm(data, addr, res, temp, "")
-#define __user_swpb_asm(data, addr, res, temp) \
-	__user_swpX_asm(data, addr, res, temp, "b")
+#define __user_swp_asm(data, addr, res, temp, temp2) \
+	__user_swpX_asm(data, addr, res, temp, temp2, "")
+#define __user_swpb_asm(data, addr, res, temp, temp2) \
+	__user_swpX_asm(data, addr, res, temp, temp2, "b")
 
 /*
  * Bit 22 of the instruction encoding distinguishes between
@@ -328,12 +336,12 @@ static int emulate_swpX(unsigned int add
 	}
 
 	while (1) {
-		unsigned long temp;
+		unsigned long temp, temp2;
 
 		if (type == TYPE_SWPB)
-			__user_swpb_asm(*data, address, res, temp);
+			__user_swpb_asm(*data, address, res, temp, temp2);
 		else
-			__user_swp_asm(*data, address, res, temp);
+			__user_swp_asm(*data, address, res, temp, temp2);
 
 		if (likely(res != -EAGAIN) || signal_pending(current))
 			break;


Patches currently in stable-queue which might be from will.deacon@arm.com are

queue-4.8/arm64-swp-emulation-bound-ll-sc-retries-before-rescheduling.patch
queue-4.8/arm64-kaslr-fix-breakage-with-config_modversions-y.patch
queue-4.8/arm64-cortex-a53-errata-workaround-check-for-kernel-addresses.patch
queue-4.8/arm64-kernel-init-mdcr_el2-even-in-the-absence-of-a-pmu.patch
queue-4.8/arm64-kvm-take-s1-walks-into-account-when-determining-s2-write-faults.patch
queue-4.8/arm64-percpu-rewrite-ll-sc-loops-in-assembly.patch