ARM: hw_breakpoint: disable preemption during debug exception handling

On ARM, debug exceptions occur in the form of data or prefetch aborts.
One difference is that debug exceptions require access to per-cpu banked
registers and data structures which are not saved in the low-level exception
code. For kernels built with CONFIG_PREEMPT, there is an unlikely scenario
that the debug handler ends up running on a different CPU from the one
that originally signalled the event, resulting in random data being read
from the wrong registers.

This patch adds a debug_entry macro to the low-level exception handling
code which checks whether the taken exception is a debug exception. If
it is, the preempt count for the faulting process is incremented. After
the debug handler has finished, the count is decremented.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 13 insertions, 5 deletions

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index d37ed3501e5..eeba3800803 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -24,6 +24,7 @@
 #define pr_fmt(fmt) "hw-breakpoint: " fmt
 
 #include <linux/errno.h>
+#include <linux/hardirq.h>
 #include <linux/perf_event.h>
 #include <linux/hw_breakpoint.h>
 #include <linux/smp.h>
@@ -736,14 +737,17 @@ unlock:
 
 /*
  * Called from either the Data Abort Handler [watchpoint] or the
- * Prefetch Abort Handler [breakpoint].
+ * Prefetch Abort Handler [breakpoint] with preemption disabled.
  */
 static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr,
                                  struct pt_regs *regs)
 {
-        int ret = 1; /* Unhandled fault. */
+        int ret = 0;
         u32 dscr;
 
+        /* We must be called with preemption disabled. */
+        WARN_ON(preemptible());
+
         /* We only handle watchpoints and hardware breakpoints. */
         ARM_DBG_READ(c1, 0, dscr);
 
@@ -758,11 +762,15 @@ static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr,
                 watchpoint_handler(addr, regs);
                 break;
         default:
-                goto out;
+                ret = 1; /* Unhandled fault. */
         }
 
-        ret = 0;
-out:
+        /*
+         * Re-enable preemption after it was disabled in the
+         * low-level exception handling code.
+         */
+        preempt_enable();
+
         return ret;
 }