[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: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/i386/kernel/kprobes.c b/arch/i386/kernel/kprobes.c
index 2f372dbd34fd..6483eeb1a4e8 100644
--- a/arch/i386/kernel/kprobes.c
+++ b/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;