[PATCH] ARM: alignment: Prevent ignoring of faults with ARMv6 unaligned access model

[Dave Martin]

Currently, it's possible to set the kernel to ignore alignment
faults when changing the alignment fault handling mode at runtime
via /proc/sys/alignment, even though this is undesirable on ARMv6
and above, where it can result in infinite spins where an un-fixed-
up instruction repeatedly faults.

In addition, the kernel clobbers any alignment mode specified on
the command-line if running on ARMv6 or above.

This patch factors out the necessary safety check into a couple of
new helper functions, and checks and modifies the fault handling
mode as appropriate on boot and on writes to /proc/cpu/alignment.

Prior to ARMv6, the behaviour is unchanged.

For ARMv6 and above, the behaviour changes as follows:

  * Attempting to ignore faults on ARMv6 results in the mode being
    forced to UM_FIXUP instead.  A warning is printed if this
    happened as a result of a write to /proc/cpu/alignment.  The
    user's UM_WARN bit (if present) is still honoured.

  * An alignment= argument from the kernel command-line is now
    honoured, except that the kernel will modify the specified mode
    as described above.  This is allows modes such as UM_SIGNAL and
    UM_WARN to be active immediately from boot, which is useful for
    debugging purposes.

Signed-off-by: Dave Martin <dave.martin at linaro.org>
---
 arch/arm/mm/alignment.c |   42 ++++++++++++++++++++++++++++++------------
 1 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 724ba3b..a55c6fc 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -95,6 +95,33 @@ static const char *usermode_action[] = {
 	"signal+warn"
 };
 
+/* Return true if and only if the ARMv6 unaligned access model is in use. */
+static bool cpu_is_v6_unaligned(void)
+{
+	return cpu_architecture() >= CPU_ARCH_ARMv6 && (cr_alignment & CR_U);
+}
+
+static int safe_usermode(int new_usermode, bool warn)
+{
+	/*
+	 * ARMv6 and later CPUs can perform unaligned accesses for
+	 * most single load and store instructions up to word size.
+	 * LDM, STM, LDRD and STRD still need to be handled.
+	 *
+	 * Ignoring the alignment fault is not an option on these
+	 * CPUs since we spin re-faulting the instruction without
+	 * making any progress.
+	 */
+	if (cpu_is_v6_unaligned() && !(new_usermode & (UM_FIXUP | UM_SIGNAL))) {
+		new_usermode |= UM_FIXUP;
+
+		if (warn)
+			printk(KERN_WARNING "alignment: ignoring faults is unsafe on this CPU.  Defaulting to fixup mode.\n");
+	}
+
+	return new_usermode;
+}
+
 static int alignment_proc_show(struct seq_file *m, void *v)
 {
 	seq_printf(m, "User:\t\t%lu\n", ai_user);
@@ -125,7 +152,7 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer
 		if (get_user(mode, buffer))
 			return -EFAULT;
 		if (mode >= '0' && mode <= '5')
-			ai_usermode = mode - '0';
+			ai_usermode = safe_usermode(mode - '0', true)
 	}
 	return count;
 }
@@ -923,20 +950,11 @@ static int __init alignment_init(void)
 		return -ENOMEM;
 #endif
 
-	/*
-	 * ARMv6 and later CPUs can perform unaligned accesses for
-	 * most single load and store instructions up to word size.
-	 * LDM, STM, LDRD and STRD still need to be handled.
-	 *
-	 * Ignoring the alignment fault is not an option on these
-	 * CPUs since we spin re-faulting the instruction without
-	 * making any progress.
-	 */
-	if (cpu_architecture() >= CPU_ARCH_ARMv6 && (cr_alignment & CR_U)) {
+	if (cpu_is_v6_unaligned()) {
 		cr_alignment &= ~CR_A;
 		cr_no_alignment &= ~CR_A;
 		set_cr(cr_alignment);
-		ai_usermode = UM_FIXUP;
+		ai_usermode = safe_usermode(ai_usermode, false);
 	}
 
 	hook_fault_code(1, do_alignment, SIGBUS, BUS_ADRALN,
-- 
1.7.4.1