kprobes/x86: Check Interrupt Flag modifier when registering probe

Currently kprobes check whether the copied instruction modifies
IF (interrupt flag) on each probe hit. This results not only in
introducing overhead but also involving
inat_get_opcode_attribute into the kprobes hot path, and it can
cause an infinite recursive call (and kernel panic in the end).

Actually, since the copied instruction itself can never be modified
on the buffer, it is needless to analyze the instruction on every
probe hit.

To fix this issue, we check it only once when registering probe
and store the result on ainsn->if_modifier.

Reported-by: Timo Juhani Lindfors <timo.lindfors@iki.fi>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: yrl.pp-manager.tt@hitachi.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20130314115242.19690.33573.stgit@mhiramat-M0-7522
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/kprobes.h

@@ -77,6 +77,7 @@ struct arch_specific_insn {
 	 * a post_handler or break_handler).
 	 */
 	int boostable;
+	bool if_modifier;
 };
 
 struct arch_optimized_insn {

arch/x86/kernel/kprobes/core.c

@@ -375,6 +375,9 @@ static void __kprobes arch_copy_kprobe(struct kprobe *p)
 	else
 		p->ainsn.boostable = -1;
 
+	/* Check whether the instruction modifies Interrupt Flag or not */
+	p->ainsn.if_modifier = is_IF_modifier(p->ainsn.insn);
+
 	/* Also, displacement change doesn't affect the first byte */
 	p->opcode = p->ainsn.insn[0];
 }
@@ -434,7 +437,7 @@ static void __kprobes set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
 	__this_cpu_write(current_kprobe, p);
 	kcb->kprobe_saved_flags = kcb->kprobe_old_flags
 		= (regs->flags & (X86_EFLAGS_TF | X86_EFLAGS_IF));
-	if (is_IF_modifier(p->ainsn.insn))
+	if (p->ainsn.if_modifier)
 		kcb->kprobe_saved_flags &= ~X86_EFLAGS_IF;
 }