From: Vasily Gorbik <gor@linux.ibm.com>

Date: Wed, 1 Mar 2023 17:58:06 +0100

Subject: s390/kprobes: fix current_kprobe never cleared after kprobes reenter

Git-commit: cd57953936f2213dfaccce10d20f396956222c7d

Patch-mainline: v6.3-rc1

References: git-fixes bsc#1211688

Recent test_kprobe_missed kprobes kunit test uncovers the following

problem. Once kprobe is triggered from another kprobe (kprobe reenter),

all future kprobes on this cpu are considered as kprobe reenter, thus

pre_handler and post_handler are not being called and kprobes are counted

as "missed".

Commit b9599798f953 ("[S390] kprobes: activation and deactivation")

introduced a simpler scheme for kprobes (de)activation and status

tracking by using push_kprobe/pop_kprobe, which supposed to work for

both initial kprobe entry as well as kprobe reentry and helps to avoid

handling those two cases differently. The problem is that a sequence of

calls in case of kprobes reenter:

push_kprobe() <- NULL (current_kprobe)

push_kprobe() <- kprobe1 (current_kprobe)

pop_kprobe() -> kprobe1 (current_kprobe)

pop_kprobe() -> kprobe1 (current_kprobe)

leaves "kprobe1" as "current_kprobe" on this cpu, instead of setting it

to NULL. In fact push_kprobe/pop_kprobe can only store a single state

(there is just one prev_kprobe in kprobe_ctlblk). Which is a hack but

sufficient, there is no need to have another prev_kprobe just to store

NULL. To make a simple and backportable fix simply reset "prev_kprobe"

when kprobe is poped from this "stack". No need to worry about

"kprobe_status" in this case, because its value is only checked when

current_kprobe != NULL.

Cc: stable@vger.kernel.org

Fixes: b9599798f953 ("[S390] kprobes: activation and deactivation")

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Acked-by: Miroslav Franc <mfranc@suse.cz>

---

 arch/s390/kernel/kprobes.c | 1 +

 1 file changed, 1 insertion(+)

diff --git a/arch/s390/kernel/kprobes.c b/arch/s390/kernel/kprobes.c

index c505c0ee5f47..e53192a5d593 100644

--- a/arch/s390/kernel/kprobes.c

+++ b/arch/s390/kernel/kprobes.c

@@ -233,6 +233,7 @@ static void pop_kprobe(struct kprobe_ctlblk *kcb)

 {

 	__this_cpu_write(current_kprobe, kcb->prev_kprobe.kp);

 	kcb->kprobe_status = kcb->prev_kprobe.status;

+	kcb->prev_kprobe.kp = NULL;

 }

 NOKPROBE_SYMBOL(pop_kprobe);