powerpc/watchpoint: Disable watchpoint hit by larx/stcx instructions

If watchpoint exception is generated by larx/stcx instructions, the
reservation created by larx gets lost while handling exception, and
thus stcx instruction always fails. Generally these instructions are
used in a while(1) loop, for example spinlocks. And because stcx
never succeeds, it loops forever and ultimately hangs the system.

Note that ptrace anyway works in one-shot mode and thus for ptrace
we don't change the behaviour. It's up to ptrace user to take care
of this.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190910131513.30499-1-ravi.bangoria@linux.ibm.com

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index 28ad317..1007ec3 100644 (file)
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -195,14 +195,32 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
        tsk->thread.last_hit_ubp = NULL;
 }
 
+static bool is_larx_stcx_instr(struct pt_regs *regs, unsigned int instr)
+{
+       int ret, type;
+       struct instruction_op op;
+
+       ret = analyse_instr(&op, regs, instr);
+       type = GETTYPE(op.type);
+       return (!ret && (type == LARX || type == STCX));
+}
+
 /*
  * Handle debug exception notifications.
  */
 static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
                             unsigned long addr)
 {
-       int stepped;
-       unsigned int instr;
+       unsigned int instr = 0;
+
+       if (__get_user_inatomic(instr, (unsigned int *)regs->nip))
+               goto fail;
+
+       if (is_larx_stcx_instr(regs, instr)) {
+               printk_ratelimited("Breakpoint hit on instruction that can't be emulated."
+                                  " Breakpoint at 0x%lx will be disabled.\n", addr);
+               goto disable;
+       }
 
        /* Do not emulate user-space instructions, instead single-step them */
        if (user_mode(regs)) {
@@ -211,23 +229,22 @@ static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,
                return false;
        }
 
-       stepped = 0;
-       instr = 0;
-       if (!__get_user_inatomic(instr, (unsigned int *)regs->nip))
-               stepped = emulate_step(regs, instr);
+       if (!emulate_step(regs, instr))
+               goto fail;
 
+       return true;
+
+fail:
        /*
-        * emulate_step() could not execute it. We've failed in reliably
-        * handling the hw-breakpoint. Unregister it and throw a warning
-        * message to let the user know about it.
+        * We've failed in reliably handling the hw-breakpoint. Unregister
+        * it and throw a warning message to let the user know about it.
         */
-       if (!stepped) {
-               WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
-                       "0x%lx will be disabled.", addr);
-               perf_event_disable_inatomic(bp);
-               return false;
-       }
-       return true;
+       WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
+               "0x%lx will be disabled.", addr);
+
+disable:
+       perf_event_disable_inatomic(bp);
+       return false;
 }
 
 int hw_breakpoint_handler(struct die_args *args)