[PATCH v2 14/15] kprobes: Remove NMI context check

Masami Hiramatsu
2020-08-27T11:53:22Z

Since the commit 9b38cc704e84 ("kretprobe: Prevent triggering
kretprobe from within kprobe_flush_task") sets a dummy current
kprobe in the trampoline handler by kprobe_busy_begin/end(),
it is not possible to run a kretprobe pre handler in kretprobe
trampoline handler context even with the NMI. If the NMI interrupts
a kretprobe_trampoline_handler() and it hits a kretprobe, the
2nd kretprobe will detect recursion correctly and it will be
skipped.
This means we have almost no double-lock issue on kretprobes by NMI.

The last one point is in cleanup_rp_inst() which also takes
kretprobe_table_lock without setting up current kprobes.
So adding kprobe_busy_begin/end() there allows us to remove
in_nmi() check.

The above commit applies kprobe_busy_begin/end() on x86, but
now all arch implementation are unified to generic one, we can
safely remove the in_nmi() check from arch independent code.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
kernel/kprobes.c | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index cbd2ad1af7b7..311033e4b8e4 100644
--- 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);
@@ -2038,17 +2041,6 @@ 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);