Patch "arm: kprobes: Align stack to 8-bytes in test code" has been added to the 4.4-stable tree

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

arm: kprobes: Align stack to 8-bytes in test code

to the 4.4-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:
arm-kprobes-align-stack-to-8-bytes-in-test-code.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jon Medhurst <[email protected]>
Date: Thu, 2 Mar 2017 13:04:09 +0000
Subject: arm: kprobes: Align stack to 8-bytes in test code

From: Jon Medhurst <[email protected]>

[ Upstream commit 974310d047f3c7788a51d10c8d255eebdb1fa857 ]

kprobes test cases need to have a stack that is aligned to an 8-byte boundary because they call other functions (and the ARM ABI mandates that alignment) and because test cases include 64-bit accesses to the stack. Unfortunately, GCC doesn't ensure this alignment for inline assembler and for the code in question seems to always misalign it by pushing just the LR register onto the stack. We therefore need to explicitly perform stack alignment at the start of each test case.

Without this fix, some test cases will generate alignment faults on systems where alignment is enforced. Even if the kernel is configured to handle these faults in software, triggering them is ugly. It also exposes limitations in the fault handling code which doesn't cope with writes to the stack. E.g. when handling this instruction

strd r6, [sp, #-64]!

the fault handling code will write to a stack location below the SP value at the point the fault occurred, which coincides with where the exception handler has pushed the saved register context. This results in corruption of those registers.

---
 arch/arm/probes/kprobes/test-core.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

@@ -976,7 +976,10 @@ static void coverage_end(void)
 void __naked __kprobes_test_case_start(void)
 {
 	__asm__ __volatile__ (
-		"stmdb	sp!, {r4-r11}				\n\t"
+		"mov	r2, sp					\n\t"
+		"bic	r3, r2, #7				\n\t"
+		"mov	sp, r3					\n\t"
+		"stmdb	sp!, {r2-r11}				\n\t"
 		"sub	sp, sp, #"__stringify(TEST_MEMORY_SIZE)"\n\t"
 		"bic	r0, lr, #1  @ r0 = inline data		\n\t"
 		"mov	r1, sp					\n\t"

@@ -996,7 +999,8 @@ void __naked __kprobes_test_case_end_32(
 		"movne	pc, r0					\n\t"
 		"mov	r0, r4					\n\t"
 		"add	sp, sp, #"__stringify(TEST_MEMORY_SIZE)"\n\t"
-		"ldmia	sp!, {r4-r11}				\n\t"
+		"ldmia	sp!, {r2-r11}				\n\t"
+		"mov	sp, r2					\n\t"
 		"mov	pc, r0					\n\t"
 	);
 }

@@ -1012,7 +1016,8 @@ void __naked __kprobes_test_case_end_16(
 		"bxne	r0					\n\t"
 		"mov	r0, r4					\n\t"
 		"add	sp, sp, #"__stringify(TEST_MEMORY_SIZE)"\n\t"
-		"ldmia	sp!, {r4-r11}				\n\t"
+		"ldmia	sp!, {r2-r11}				\n\t"
+		"mov	sp, r2					\n\t"
 		"bx	r0					\n\t"
 	);
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/arm-kprobes-fix-the-return-address-of-multiple-kretprobes.patch queue-4.4/arm-kprobes-align-stack-to-8-bytes-in-test-code.patch