ARM: hw_breakpoint: disallow per-cpu breakpoints without overflow handler

Single-stepping a breakpoint requires us to disable it temporarily so that
we don't get stuck in a recursive debug trap. With per-cpu breakpoints this
presents a problem where an interrupt can be taken before the single-step has
completed and a new task is eventually scheduled. This new task will not
hit the breakpoint because it will have been disabled during the previous
handling code.

This patch disallows per-cpu breakpoints on ARM when an overflow handler
is not present. A similar effect can be created by placing breakpoints on
a shell and then running applications there.

Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index 36cd7680d3d..eef1b1e235a 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -622,10 +622,12 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
          * Currently we rely on an overflow handler to take
          * care of single-stepping the breakpoint when it fires.
          * In the case of userspace breakpoints on a core with V7 debug,
-         * we can use the mismatch feature as a poor-man's hardware single-step.
+         * we can use the mismatch feature as a poor-man's hardware
+         * single-step, but this only works for per-task breakpoints.
          */
         if (WARN_ONCE(!bp->overflow_handler &&
-                (arch_check_bp_in_kernelspace(bp) || !core_has_mismatch_brps()),
+                (arch_check_bp_in_kernelspace(bp) || !core_has_mismatch_brps()
+                 || !bp->hw.bp_target),
                         "overflow handler required but none found")) {
                 ret = -EINVAL;
         }