Patch "powerpc: Don't try to fix up misaligned load-with-reservation instructions" 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

powerpc: Don't try to fix up misaligned load-with-reservation instructions

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:
powerpc-don-t-try-to-fix-up-misaligned-load-with-reservation-instructions.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: Paul Mackerras <[email protected]>
Date: Tue, 4 Apr 2017 14:56:05 +1000
Subject: powerpc: Don't try to fix up misaligned load-with-reservation instructions

From: Paul Mackerras <[email protected]>

commit 48fe9e9488743eec9b7c1addd3c93f12f2123d54 upstream.

In the past, there was only one load-with-reservation instruction, lwarx, and if a program attempted a lwarx on a misaligned address, it would take an alignment interrupt and the kernel handler would emulate it as though it was lwzx, which was not really correct, but benign since it is loading the right amount of data, and the lwarx should be paired with a stwcx. to the same address, which would also cause an alignment interrupt which would result in a SIGBUS being delivered to the process.

We now have 5 different sizes of load-with-reservation instruction. Of those, lharx and ldarx cause an immediate SIGBUS by luck since their entries in aligninfo[] overlap instructions which were not fixed up, but lqarx overlaps with lhz and will be emulated as such. lbarx can never generate an alignment interrupt since it only operates on 1 byte.

To straighten this out and fix the lqarx case, this adds code to detect the l[hwdq]arx instructions and return without fixing them up, resulting in a SIGBUS being delivered to the process.

---
 arch/powerpc/kernel/align.c |   27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

@@ -808,14 +808,25 @@ int fix_alignment(struct pt_regs *regs)
 	nb = aligninfo[instr].len;
 	flags = aligninfo[instr].flags;
 
-	/* ldbrx/stdbrx overlap lfs/stfs in the DSISR unfortunately */
-	if (IS_XFORM(instruction) && ((instruction >> 1) & 0x3ff) == 532) {
-		nb = 8;
-		flags = LD+SW;
-	} else if (IS_XFORM(instruction) &&
-		   ((instruction >> 1) & 0x3ff) == 660) {
-		nb = 8;
-		flags = ST+SW;
+	/*
+	 * Handle some cases which give overlaps in the DSISR values.
+	 */
+	if (IS_XFORM(instruction)) {
+		switch (get_xop(instruction)) {
+		case 532:	/* ldbrx */
+			nb = 8;
+			flags = LD+SW;
+			break;
+		case 660:	/* stdbrx */
+			nb = 8;
+			flags = ST+SW;
+			break;
+		case 20:	/* lwarx */
+		case 84:	/* ldarx */
+		case 116:	/* lharx */
+		case 276:	/* lqarx */
+			return 0;	/* not emulated ever */
+		}
 	}
 
 	/* Byteswap little endian loads and stores */

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

queue-3.18/powerpc-don-t-try-to-fix-up-misaligned-load-with-reservation-instructions.patch