Patch "powerpc/kprobe: Fix oops when kprobed on 'stdu' instruction" has been added to the 4.9-stable tree

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

powerpc/kprobe: Fix oops when kprobed on 'stdu' instruction

to the 4.9-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:
powerpc-kprobe-fix-oops-when-kprobed-on-stdu-instruction.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Ravi Bangoria <[email protected]>
Date: Tue, 11 Apr 2017 10:38:13 +0530
Subject: powerpc/kprobe: Fix oops when kprobed on 'stdu' instruction

From: Ravi Bangoria <[email protected]>

commit 9e1ba4f27f018742a1aa95d11e35106feba08ec1 upstream.

If we set a kprobe on a 'stdu' instruction on powerpc64, we see a kernel OOPS:

Bad kernel stack pointer cd93c840 at c000000000009868

  Oops: Bad kernel stack pointer, sig: 6 [#1]
  ...
  GPR00: c000001fcd93cb30 00000000cd93c840 c0000000015c5e00 00000000cd93c840
  ...
  NIP [c000000000009868] resume_kernel+0x2c/0x58
  LR [c000000000006208] program_check_common+0x108/0x180

On a 64-bit system when the user probes on a 'stdu' instruction, the kernel does not emulate actual store in emulate_step() because it may corrupt the exception frame. So the kernel does the actual store operation in exception return code i.e. resume_kernel().

resume_kernel() loads the saved stack pointer from memory using lwz, which only loads the low 32-bits of the address, causing the kernel crash.

Fix this by loading the 64-bit value instead.

---
 arch/powerpc/kernel/entry_64.S |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

@@ -689,7 +689,7 @@ resume_kernel:
 
 	addi	r8,r1,INT_FRAME_SIZE	/* Get the kprobed function entry */
 
-	lwz	r3,GPR1(r1)
+	ld	r3,GPR1(r1)
 	subi	r3,r3,INT_FRAME_SIZE	/* dst: Allocate a trampoline exception frame */
 	mr	r4,r1			/* src:  current exception frame */
 	mr	r1,r3			/* Reroute the trampoline frame to r1 */

@@ -703,8 +703,8 @@ resume_kernel:
 	addi	r6,r6,8
 	bdnz	2b
 
-	/* Do real store operation to complete stwu */
-	lwz	r5,GPR1(r1)
+	/* Do real store operation to complete stdu */
+	ld	r5,GPR1(r1)
 	std	r8,0(r5)
 
 	/* Clear _TIF_EMULATE_STACK_STORE flag */

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

queue-4.9/powerpc-kprobe-fix-oops-when-kprobed-on-stdu-instruction.patch