[PATCH v2 2/2] MIPS: ptrace: disable watchpoints if hit in kernel mode

[Marcin Nowakowski <marcin.nowakowski@imgtec.com>]

If a watchpoint is hit when in kernel mode it is possible for the system
to end up in an infinite loop processing the watchpoint. This can happen
if a user sets a watchpoint in the kernel addess space (which is
possible in certain EVA configurations) or if a user sets a watchpoint
in a user area accessed directly by the kernel (eg. a user buffer
accessed via a syscall).

To prevent the infinite loop ensure that the watchpoint was hit in
userspace, and clear the watchpoint registers otherwise.

As this change could mean that a watchpoint is not hit when it should be
(when returning to the interrupted traced task on exception exit), the
resume_userspace path needs to be extended to conditionally restore the
watchpoint configuration. If a task switch occurs when returning to
userspace, the watchpoints will be restored in a typical way in the
switch_to() handler.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
---
 arch/mips/kernel/entry.S | 9 ++++++++-
 arch/mips/kernel/traps.c | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
index 7791840..1982fc6 100644
--- a/arch/mips/kernel/entry.S
+++ b/arch/mips/kernel/entry.S
@@ -55,7 +55,14 @@ resume_userspace:
 	LONG_L	a2, TI_FLAGS($28)	# current->work
 	andi	t0, a2, _TIF_WORK_MASK	# (ignoring syscall_trace)
 	bnez	t0, work_pending
-	j	restore_all
+#ifdef CONFIG_HARDWARE_WATCHPOINTS
+	li	t0, _TIF_LOAD_WATCH
+	and	t0, a2, t0
+	beqz	t0, 1f
+	PTR_L	a0, TI_TASK($28)
+	jal	mips_install_watch_registers
+#endif
+1:	j	restore_all
 
 #ifdef CONFIG_PREEMPT
 resume_kernel:
diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 6c7f9d7..5371cb0 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -1525,7 +1525,7 @@ asmlinkage void do_watch(struct pt_regs *regs)
 	 * their values and send SIGTRAP.  Otherwise another thread
 	 * left the registers set, clear them and continue.
 	 */
-	if (test_tsk_thread_flag(current, TIF_LOAD_WATCH)) {
+	if (user_mode(regs) && test_tsk_thread_flag(current, TIF_LOAD_WATCH)) {
 		mips_read_watch_registers();
 		local_irq_enable();
 		force_sig_info(SIGTRAP, &info, current);
-- 
2.7.4