[PATCH] LoongArch: kprobes: Avoid preempt count underflow without probe

LoongArch uses break 11 for the breakpoint placed after an instruction
that Kprobes executes out of line. Since userspace can issue the same
break instruction, do_bp() can reach kprobe_singlestep_handler() when
there is no current probe.

The handler returns false in this case, but first calls
preempt_enable_no_resched(). The corresponding preempt_disable() is done
by kprobe_breakpoint_handler() on a real Kprobe hit, so it has not run
here. As a result, an ordinary userspace breakpoint underflows the
current task's preempt count.

This also makes in_interrupt() return true until the task schedules. One
visible consequence is socket cgroup attribution: cgroup_sk_alloc()
treats the allocation as interrupt context and assigns the socket to the
root cgroup. A socket opened from the SIGTRAP handler can then avoid a
BPF_CGROUP_INET_SOCK_CREATE policy attached to the task's own cgroup.

Return as soon as kprobe_running() reports no active probe.

The same check appeared in [PATCH v10 2/4] of the original LoongArch
Kprobes series, but was dropped before the feature reached mainline.

Link: https://patchew.org/linux/1670575981-14389-1-git-send-email-yangtiezhu%40loongson.cn/1670575981-14389-3-git-send-email-yangtiezhu%40loongson.cn/
Fixes: 6d4cc40fb5f5 ("LoongArch: Add kprobes support")
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <[email protected]>
---
 arch/loongarch/kernel/kprobes.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/loongarch/kernel/kprobes.c b/arch/loongarch/kernel/kprobes.c
index 1985ed30dd16f..ddfefea174727 100644
--- a/arch/loongarch/kernel/kprobes.c
+++ b/arch/loongarch/kernel/kprobes.c
@@ -275,6 +275,9 @@ bool kprobe_singlestep_handler(struct pt_regs *regs)
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 	unsigned long addr = instruction_pointer(regs);
 
+	if (!cur)
+		return false;
+
 	if (cur && (kcb->kprobe_status & (KPROBE_HIT_SS | KPROBE_REENTER)) &&
 	    ((unsigned long)&cur->ainsn.insn[1] == addr)) {
 		restore_local_irqflag(kcb, regs);
-- 
2.47.3