x86: fix singlestep handling in reenter_kprobe

Highlight peculiar cases in singles-step kprobe handling.

In reenter_kprobe(), a breakpoint in KPROBE_HIT_SS case can only occur
when single-stepping a breakpoint on which a probe was installed. Since
such probes are single-stepped inline, identifying these cases is
unambiguous. All other cases leading up to KPROBE_HIT_SS are possible
bugs. Identify and WARN_ON such cases.

Signed-off-by: Abhishek Sagar <sagar.abhishek@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 13 insertions, 15 deletions

diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index edc0a8e59247..f7ddbb8c3fe8 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -443,17 +443,6 @@ void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
         *sara = (unsigned long) &kretprobe_trampoline;
 }
 
-static void __kprobes recursive_singlestep(struct kprobe *p,
-                                           struct pt_regs *regs,
-                                           struct kprobe_ctlblk *kcb)
-{
-        save_previous_kprobe(kcb);
-        set_current_kprobe(p, regs, kcb);
-        kprobes_inc_nmissed_count(p);
-        prepare_singlestep(p, regs);
-        kcb->kprobe_status = KPROBE_REENTER;
-}
-
 static void __kprobes setup_singlestep(struct kprobe *p, struct pt_regs *regs,
                                        struct kprobe_ctlblk *kcb)
 {
@@ -492,20 +481,29 @@ static int __kprobes reenter_kprobe(struct kprobe *p, struct pt_regs *regs,
                 break;
 #endif
         case KPROBE_HIT_ACTIVE:
-                recursive_singlestep(p, regs, kcb);
+                save_previous_kprobe(kcb);
+                set_current_kprobe(p, regs, kcb);
+                kprobes_inc_nmissed_count(p);
+                prepare_singlestep(p, regs);
+                kcb->kprobe_status = KPROBE_REENTER;
                 break;
         case KPROBE_HIT_SS:
-                if (*p->ainsn.insn == BREAKPOINT_INSTRUCTION) {
+                if (p == kprobe_running()) {
                         regs->flags &= ~TF_MASK;
                         regs->flags |= kcb->kprobe_saved_flags;
                         return 0;
                 } else {
-                        recursive_singlestep(p, regs, kcb);
+                        /* A probe has been hit in the codepath leading up
+                         * to, or just after, single-stepping of a probed
+                         * instruction. This entire codepath should strictly
+                         * reside in .kprobes.text section. Raise a warning
+                         * to highlight this peculiar case.
+                         */
                 }
-                break;
         default:
                 /* impossible cases */
                 WARN_ON(1);
+                return 0;
         }
 
         return 1;