[BUGFIX PATCH -tip] x86/kprobes: Fix to call previous kprobe's fault handler

[Masami Hiramatsu]

Fix to call the previous kprobe's fault handler when a page fault
occurred in reentered kprobes.
This may happen if kprobes handler calls a function which can cause
a page fault (e.g. access user page) and another kprobe probes that
instruction.
Without the 2nd kprobe, the 1st kprobe can handle the page fault,
but with the 2nd kprobe, the 1st one can not handle it.

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---
 arch/x86/kernel/kprobes/core.c |   19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 7c4d0736a998..ac2514f1e195 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -1098,12 +1098,21 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		 */
 		regs->flags |= kcb->kprobe_old_flags;
 
-		if (kcb->kprobe_status == KPROBE_REENTER)
-			restore_previous_kprobe(kcb);
-		else
+		if (kcb->kprobe_status != KPROBE_REENTER) {
 			reset_current_kprobe();
-	} else if (kcb->kprobe_status == KPROBE_HIT_ACTIVE ||
-		   kcb->kprobe_status == KPROBE_HIT_SSDONE) {
+			return 0;
+		}
+		restore_previous_kprobe(kcb);
+		/*
+		 * If reentered kprobes caused a page fault, it must be
+		 * handled by the previous kprobe too. But we don't bother
+		 * checking KPROBE_HIT_SS again because kprobes can not
+		 * probe another kprobe's single stepping buffer.
+		 */
+	}
+
+	if (kcb->kprobe_status == KPROBE_HIT_ACTIVE ||
+	    kcb->kprobe_status == KPROBE_HIT_SSDONE) {
 		/*
 		 * We increment the nmissed count for accounting,
 		 * we can also use npre/npostfault count for accounting