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, 19 insertions, 0 deletions

diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index d93f976fb389..ae9464900168 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -165,6 +165,25 @@
         .endm
 #endif  /* !CONFIG_THUMB2_KERNEL */
 
+        @
+        @ Debug exceptions are taken as prefetch or data aborts.
+        @ We must disable preemption during the handler so that
+        @ we can access the debug registers safely.
+        @
+        .macro  debug_entry, fsr
+#if defined(CONFIG_HAVE_HW_BREAKPOINT) && defined(CONFIG_PREEMPT)
+        ldr     r4, =0x40f              @ mask out fsr.fs
+        and     r5, r4, \fsr
+        cmp     r5, #2                  @ debug exception
+        bne     1f
+        get_thread_info r10
+        ldr     r6, [r10, #TI_PREEMPT]  @ get preempt count
+        add     r11, r6, #1             @ increment it
+        str     r11, [r10, #TI_PREEMPT]
+1:
+#endif
+        .endm
+
 /*
  * These are the registers used in the syscall handler, and allow us to
  * have in theory up to 7 arguments to a function - r0 to r6.