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, 4 insertions, 0 deletions

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index c09e3573c5de..34bbef0d2e7e 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -198,6 +198,7 @@ __dabt_svc:
         @
         @ set desired IRQ state, then call main handler
         @
+        debug_entry r1
         msr     cpsr_c, r9
         mov     r2, sp
         bl      do_DataAbort
@@ -324,6 +325,7 @@ __pabt_svc:
 #else
         bl      CPU_PABORT_HANDLER
 #endif
+        debug_entry r1
         msr     cpsr_c, r9                      @ Maybe enable interrupts
         mov     r2, sp                          @ regs
         bl      do_PrefetchAbort                @ call abort handler
@@ -439,6 +441,7 @@ __dabt_usr:
         @
         @ IRQs on, then call the main handler
         @
+        debug_entry r1
         enable_irq
         mov     r2, sp
         adr     lr, BSYM(ret_from_exception)
@@ -703,6 +706,7 @@ __pabt_usr:
 #else
         bl      CPU_PABORT_HANDLER
 #endif
+        debug_entry r1
         enable_irq                              @ Enable interrupts
         mov     r2, sp                          @ regs
         bl      do_PrefetchAbort                @ call abort handler