Patch "arm64: entry: improve data abort handling of tagged pointers" has been added to the 3.18-stable tree

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

arm64: entry: improve data abort handling of tagged pointers

to the 3.18-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-entry-improve-data-abort-handling-of-tagged-pointers.patch
and it can be found in the queue-3.18 subdirectory.

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

From: Kristina Martsenko <[email protected]>
Date: Wed, 3 May 2017 16:37:47 +0100
Subject: arm64: entry: improve data abort handling of tagged pointers

From: Kristina Martsenko <[email protected]>

commit 276e93279a630657fff4b086ba14c95955912dfa upstream.

When handling a data abort from EL0, we currently zero the top byte of the faulting address, as we assume the address is a TTBR0 address, which may contain a non-zero address tag. However, the address may be a TTBR1 address, in which case we should not zero the top byte. This patch fixes that. The effect is that the full TTBR1 address is passed to the task's signal handler (or printed out in the kernel log).

When handling a data abort from EL1, we leave the faulting address intact, as we assume it's either a TTBR1 address or a TTBR0 address with tag 0x00. This is true as far as I'm aware, we don't seem to access a tagged TTBR0 address anywhere in the kernel. Regardless, it's easy to forget about address tags, and code added in the future may not always remember to remove tags from addresses before accessing them. So add tag handling to the EL1 data abort handler as well. This also makes it consistent with the EL0 data abort handler.

---
 arch/arm64/include/asm/asm-uaccess.h |   13 +++++++++++++
 arch/arm64/kernel/entry.S            |    6 ++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

@@ -0,0 +1,13 @@
+#ifndef __ASM_ASM_UACCESS_H
+#define __ASM_ASM_UACCESS_H
+
+/*
+ * Remove the address tag from a virtual address, if present.
+ */
+	.macro	clear_address_tag, dst, addr
+	tst	\addr, #(1 << 55)
+	bic	\dst, \addr, #(0xff << 56)
+	csel	\dst, \dst, \addr, eq
+	.endm
+
+#endif

@@ -28,6 +28,7 @@
 #include <asm/errno.h>
 #include <asm/esr.h>
 #include <asm/thread_info.h>
+#include <asm/asm-uaccess.h>
 #include <asm/unistd.h>
 
 /*

@@ -307,12 +308,13 @@ el1_da:
 	/*
 	 * Data abort handling
 	 */
-	mrs	x0, far_el1
+	mrs	x3, far_el1
 	enable_dbg
 	// re-enable interrupts if they were enabled in the aborted context
 	tbnz	x23, #7, 1f			// PSR_I_BIT
 	enable_irq
 1:
+	clear_address_tag x0, x3
 	mov	x2, sp				// struct pt_regs
 	bl	do_mem_abort
 

@@ -472,7 +474,7 @@ el0_da:
 	// enable interrupts before calling the main handler
 	enable_dbg_and_irq
 	ct_user_exit
-	bic	x0, x26, #(0xff << 56)
+	clear_address_tag x0, x26
 	mov	x1, x25
 	mov	x2, sp
 	adr	lr, ret_to_user

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

queue-3.18/arm64-hw_breakpoint-fix-watchpoint-matching-for-tagged-pointers.patch queue-3.18/arm64-entry-improve-data-abort-handling-of-tagged-pointers.patch