kprobes: Remove NMI context check

The in_nmi() check in pre_handler_kretprobe() is meant to avoid
recursion, and blindly assumes that anything NMI is recursive.

However, since commit:

  9b38cc704e84 ("kretprobe: Prevent triggering kretprobe from within kprobe_flush_task")

there is a better way to detect and avoid actual recursion.

By setting a dummy kprobe, any actual exceptions will terminate early
(by trying to handle the dummy kprobe), and recursion will not happen.

Employ this to avoid the kretprobe_table_lock() recursion, replacing
the over-eager in_nmi() check.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/159870615628.1229682.6087311596892125907.stgit@devnote2

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index a0afaa7..2111382 100644 (file)
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1359,7 +1359,8 @@ static void cleanup_rp_inst(struct kretprobe *rp)
        struct hlist_node *next;
        struct hlist_head *head;
 
-       /* No race here */
+       /* To avoid recursive kretprobe by NMI, set kprobe busy here */
+       kprobe_busy_begin();
        for (hash = 0; hash < KPROBE_TABLE_SIZE; hash++) {
                kretprobe_table_lock(hash, &flags);
                head = &kretprobe_inst_table[hash];
@@ -1369,6 +1370,8 @@ static void cleanup_rp_inst(struct kretprobe *rp)
                }
                kretprobe_table_unlock(hash, &flags);
        }
+       kprobe_busy_end();
+
        free_rp_inst(rp);
 }
 NOKPROBE_SYMBOL(cleanup_rp_inst);
@@ -2035,17 +2038,6 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
        unsigned long hash, flags = 0;
        struct kretprobe_instance *ri;
 
-       /*
-        * To avoid deadlocks, prohibit return probing in NMI contexts,
-        * just skip the probe and increase the (inexact) 'nmissed'
-        * statistical counter, so that the user is informed that
-        * something happened:
-        */
-       if (unlikely(in_nmi())) {
-               rp->nmissed++;
-               return 0;
-       }
-
        /* TODO: consider to only swap the RA after the last pre_handler fired */
        hash = hash_ptr(current, KPROBE_HASH_BITS);
        raw_spin_lock_irqsave(&rp->lock, flags);