Patch "arm64: mm: Set PSTATE.PAN from the cpu_enable_pan() call" 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

arm64: mm: Set PSTATE.PAN from the cpu_enable_pan() call

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:
arm64-mm-set-pstate.pan-from-the-cpu_enable_pan-call.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: James Morse <[email protected]>
Date: Tue, 18 Oct 2016 11:27:47 +0100
Subject: arm64: mm: Set PSTATE.PAN from the cpu_enable_pan() call

From: James Morse <[email protected]>

commit 7209c868600bd8926e37c10b9aae83124ccc1dd8 upstream.

Commit 338d4f49d6f7 ("arm64: kernel: Add support for Privileged Access Never") enabled PAN by enabling the 'SPAN' feature-bit in SCTLR_EL1. This means the PSTATE.PAN bit won't be set until the next return to the kernel from userspace. On a preemptible kernel we may schedule work that accesses userspace on a CPU before it has done this.

Now that cpufeature enable() calls are scheduled via stop_machine(), we can set PSTATE.PAN from the cpu_enable_pan() call.

Add WARN_ON_ONCE(in_interrupt()) to check the PSTATE value we updated is not immediately discarded.

---
 arch/arm64/mm/fault.c |    9 +++++++++
 1 file changed, 9 insertions(+)

@@ -29,7 +29,9 @@
 #include <linux/sched.h>
 #include <linux/highmem.h>
 #include <linux/perf_event.h>
+#include <linux/preempt.h>
 
+#include <asm/bug.h>
 #include <asm/cpufeature.h>
 #include <asm/exception.h>
 #include <asm/debug-monitors.h>

@@ -608,7 +610,14 @@ asmlinkage int __exception do_debug_exce
 #ifdef CONFIG_ARM64_PAN
 int cpu_enable_pan(void *__unused)
 {
+	/*
+	 * We modify PSTATE. This won't work from irq context as the PSTATE
+	 * is discarded once we return from the exception.
+	 */
+	WARN_ON_ONCE(in_interrupt());
+
 	config_sctlr_el1(SCTLR_EL1_SPAN, 0);
+	asm(SET_PSTATE_PAN(1));
 	return 0;
 }
 #endif /* CONFIG_ARM64_PAN */

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

queue-4.4/arm64-mm-set-pstate.pan-from-the-cpu_enable_pan-call.patch queue-4.4/arm64-suspend-reconfigure-pstate-after-resume-from-idle.patch queue-4.4/arm64-cpufeature-schedule-enable-calls-instead-of-calling-them-via-ipi.patch