[PATCH] kprobes: fix race in recovery of reentrant probe

There is a window where a probe gets removed right after the probe is hit
on some different cpu.  In this case probe handlers can't find a matching
probe instance related to break address.  In this case we need to read the
original instruction at break address to see if that is not a break/int3
instruction and recover safely.

Previous code had a bug where we were not checking for the above race in
case of reentrant probes and the below patch fixes this race.

Tested on IA64, Powerpc, x86_64.
Signed-off-by: NAnil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/i386/kernel/kprobes.c

@@ -188,6 +188,19 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 			kcb->kprobe_status = KPROBE_REENTER;
 			return 1;
 		} else {
+			if (regs->eflags & VM_MASK) {
+			/* We are in virtual-8086 mode. Return 0 */
+				goto no_kprobe;
+			}
+			if (*addr != BREAKPOINT_INSTRUCTION) {
+			/* The breakpoint instruction was removed by
+			 * another cpu right after we hit, no further
+			 * handling of this interrupt is appropriate
+			 */
+				regs->eip -= sizeof(kprobe_opcode_t);
+				ret = 1;
+				goto no_kprobe;
+			}
 			p = __get_cpu_var(current_kprobe);
 			if (p->break_handler && p->break_handler(p, regs)) {
 				goto ss_probe;

arch/ia64/kernel/kprobes.c

@@ -638,6 +638,13 @@ static int __kprobes pre_kprobes_handler(struct die_args *args)
 			if (p->break_handler && p->break_handler(p, regs)) {
 				goto ss_probe;
 			}
+		} else if (!is_ia64_break_inst(regs)) {
+			/* The breakpoint instruction was removed by
+			 * another cpu right after we hit, no further
+			 * handling of this interrupt is appropriate
+			 */
+			ret = 1;
+			goto no_kprobe;
 		} else {
 			/* Not our break */
 			goto no_kprobe;

arch/powerpc/kernel/kprobes.c

@@ -179,6 +179,18 @@ static inline int kprobe_handler(struct pt_regs *regs)
 			kcb->kprobe_status = KPROBE_REENTER;
 			return 1;
 		} else {
+			if (*addr != BREAKPOINT_INSTRUCTION) {
+				/* If trap variant, then it belongs not to us */
+				kprobe_opcode_t cur_insn = *addr;
+				if (is_trap(cur_insn))
+		       			goto no_kprobe;
+				/* The breakpoint instruction was removed by
+				 * another cpu right after we hit, no further
+				 * handling of this interrupt is appropriate
+				 */
+				ret = 1;
+				goto no_kprobe;
+			}
 			p = __get_cpu_var(current_kprobe);
 			if (p->break_handler && p->break_handler(p, regs)) {
 				goto ss_probe;

arch/sparc64/kernel/kprobes.c

@@ -135,6 +135,14 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 			prepare_singlestep(p, regs, kcb);
 			return 1;
 		} else {
+			if (*(u32 *)addr != BREAKPOINT_INSTRUCTION) {
+			/* The breakpoint instruction was removed by
+			 * another cpu right after we hit, no further
+			 * handling of this interrupt is appropriate
+			 */
+				ret = 1;
+				goto no_kprobe;
+			}
 			p = __get_cpu_var(current_kprobe);
 			if (p->break_handler && p->break_handler(p, regs))
 				goto ss_probe;

arch/x86_64/kernel/kprobes.c

@@ -334,6 +334,15 @@ int __kprobes kprobe_handler(struct pt_regs *regs)
 				return 1;
 			}
 		} else {
+			if (*addr != BREAKPOINT_INSTRUCTION) {
+			/* The breakpoint instruction was removed by
+			 * another cpu right after we hit, no further
+			 * handling of this interrupt is appropriate
+			 */
+				regs->rip = (unsigned long)addr;
+				ret = 1;
+				goto no_kprobe;
+			}
 			p = __get_cpu_var(current_kprobe);
 			if (p->break_handler && p->break_handler(p, regs)) {
 				goto ss_probe;