Patch "x86/alternatives: Make optimize_nops() interrupt safe and synced" has been added to the 4.2-stable tree

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

x86/alternatives: Make optimize_nops() interrupt safe and synced

to the 4.2-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:
x86-alternatives-make-optimize_nops-interrupt-safe-and-synced.patch
and it can be found in the queue-4.2 subdirectory.

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

From: Thomas Gleixner <[email protected]>
Date: Thu, 3 Sep 2015 12:34:55 +0200
Subject: x86/alternatives: Make optimize_nops() interrupt safe and synced

From: Thomas Gleixner <[email protected]>

commit 66c117d7fa2ae429911e60d84bf31a90b2b96189 upstream.

Richard reported the following crash:

[    0.036000] BUG: unable to handle kernel paging request at 55501e06
[    0.036000] IP: [<c0aae48b>] common_interrupt+0xb/0x38
[    0.036000] Call Trace:
[    0.036000]  [<c0409c80>] ? add_nops+0x90/0xa0
[    0.036000]  [<c040a054>] apply_alternatives+0x274/0x630

Chuck decoded:

 "  0:   8d 90 90 83 04 24       lea    0x24048390(%eax),%edx
    6:   80 fc 0f                cmp    $0xf,%ah
    9:   a8 0f                   test   $0xf,%al
 >> b:   a0 06 1e 50 55          mov    0x55501e06,%al
   10:   57                      push   %edi
   11:   56                      push   %esi

Interrupt 0x30 occurred while the alternatives code was replacing the initial 0x90,0x90,0x90 NOPs (from the ASM_CLAC macro) with the optimized version, 0x8d,0x76,0x00. Only the first byte has been replaced so far, and it makes a mess out of the insn decoding."

optimize_nops() is buggy in two aspects:

- It's not disabling interrupts across the modification
- It's lacking a sync_core() call

Add both.

---
 arch/x86/kernel/alternative.c |    5 +++++
 1 file changed, 5 insertions(+)

@@ -338,10 +338,15 @@ done:
 
 static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr)
 {
+	unsigned long flags;
+
 	if (instr[0] != 0x90)
 		return;
 
+	local_irq_save(flags);
 	add_nops(instr + (a->instrlen - a->padlen), a->padlen);
+	sync_core();
+	local_irq_restore(flags);
 
 	DUMP_BYTES(instr, a->instrlen, "%p: [%d:%d) optimized NOPs: ",
 		   instr, a->instrlen - a->padlen, a->padlen);

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

queue-4.2/time-fix-timekeeping_freqadjust-s-incorrect-use-of-abs-instead-of-abs64.patch queue-4.2/x86-platform-fix-geode-lx-timekeeping-in-the-generic-x86-build.patch queue-4.2/x86-pci-intel_mid_pci-work-around-for-irq0-assignment.patch queue-4.2/x86-ioapic-force-affinity-setting-in-setup_ioapic_dest.patch queue-4.2/x86-alternatives-make-optimize_nops-interrupt-safe-and-synced.patch queue-4.2/x86-apic-serialize-lvtt-and-tsc_deadline-writes.patch queue-4.2/perf-x86-intel-fix-constraint-access.patch queue-4.2/locking-qspinlock-x86-only-emit-the-test-and-set-fallback-when-building-guest-support.patch queue-4.2/locking-qspinlock-x86-fix-performance-regression-under-unaccelerated-vms.patch