[tip:x86/core] x86/kprobes: Refactor kprobes_fault() like kprobe_exceptions_notify()

[tip-bot for Jann Horn <tipbot@zytor.com>]

Commit-ID:  a980c0ef9f6d8c45445d6ed0f5836bb6941c8c91
Gitweb:     https://git.kernel.org/tip/a980c0ef9f6d8c45445d6ed0f5836bb6941c8c91
Author:     Jann Horn <jannh@google.com>
AuthorDate: Tue, 28 Aug 2018 22:14:15 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Mon, 3 Sep 2018 15:12:08 +0200

x86/kprobes: Refactor kprobes_fault() like kprobe_exceptions_notify()

This is an extension of commit b506a9d08bae ("x86: code clarification patch
to Kprobes arch code"). As that commit explains, even though
kprobe_running() can't be called with preemption enabled, preemption does
not need to be disabled. If preemption is enabled, then this can't be
originate from a kprobe.

Also, use X86_TRAP_PF instead of 14.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kees Cook <keescook@chromium.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: kernel-hardening@lists.openwall.com
Cc: dvyukov@google.com
Cc: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/20180828201421.157735-2-jannh@google.com

---
 arch/x86/mm/fault.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 47bebfe6efa7..d99edb2d567b 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -44,17 +44,19 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr)
 
 static nokprobe_inline int kprobes_fault(struct pt_regs *regs)
 {
-	int ret = 0;
-
-	/* kprobe_running() needs smp_processor_id() */
-	if (kprobes_built_in() && !user_mode(regs)) {
-		preempt_disable();
-		if (kprobe_running() && kprobe_fault_handler(regs, 14))
-			ret = 1;
-		preempt_enable();
-	}
-
-	return ret;
+	if (!kprobes_built_in())
+		return 0;
+	if (user_mode(regs))
+		return 0;
+	/*
+	 * To be potentially processing a kprobe fault and to be allowed to call
+	 * kprobe_running(), we have to be non-preemptible.
+	 */
+	if (preemptible())
+		return 0;
+	if (!kprobe_running())
+		return 0;
+	return kprobe_fault_handler(regs, X86_TRAP_PF);
 }
 
 /*