Re: [PATCH v5 14/21] kprobes: Remove NMI context check

[Steven Rostedt]

On Wed, 4 Nov 2020 11:08:52 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> kretprobe_hash_lock() and kretprobe_table_lock() will be called from
> outside of the kprobe pre_handler context. So, please keep in_nmi()
> in those functions.
> for the pre_handler_kretprobe(), this looks good to me.
> 

Final version, before sending to Linus.

-- Steve

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
Subject: [PATCH] kprobes: Tell lockdep about kprobe nesting

Since the kprobe handlers have protection that prohibits other handlers from
executing in other contexts (like if an NMI comes in while processing a
kprobe, and executes the same kprobe, it will get fail with a "busy"
return). Lockdep is unaware of this protection. Use lockdep's nesting api to
differentiate between locks taken in INT3 context and other context to
suppress the false warnings.

Link: https://lore.kernel.org/r/20201102160234.fa0ae70915ad9e2b21c08b85@xxxxxxxxxx

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 kernel/kprobes.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 8a12a25fa40d..41fdbb7953c6 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1249,7 +1249,13 @@ __acquires(hlist_lock)
 
 	*head = &kretprobe_inst_table[hash];
 	hlist_lock = kretprobe_table_lock_ptr(hash);
-	raw_spin_lock_irqsave(hlist_lock, *flags);
+	/*
+	 * Nested is a workaround that will soon not be needed.
+	 * There's other protections that make sure the same lock
+	 * is not taken on the same CPU that lockdep is unaware of.
+	 * Differentiate when it is taken in NMI context.
+	 */
+	raw_spin_lock_irqsave_nested(hlist_lock, *flags, !!in_nmi());
 }
 NOKPROBE_SYMBOL(kretprobe_hash_lock);
 
@@ -1258,7 +1264,13 @@ static void kretprobe_table_lock(unsigned long hash,
 __acquires(hlist_lock)
 {
 	raw_spinlock_t *hlist_lock = kretprobe_table_lock_ptr(hash);
-	raw_spin_lock_irqsave(hlist_lock, *flags);
+	/*
+	 * Nested is a workaround that will soon not be needed.
+	 * There's other protections that make sure the same lock
+	 * is not taken on the same CPU that lockdep is unaware of.
+	 * Differentiate when it is taken in NMI context.
+	 */
+	raw_spin_lock_irqsave_nested(hlist_lock, *flags, !!in_nmi());
 }
 NOKPROBE_SYMBOL(kretprobe_table_lock);
 
@@ -2028,7 +2040,12 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 
 	/* 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);
+	/*
+	 * Nested is a workaround that will soon not be needed.
+	 * There's other protections that make sure the same lock
+	 * is not taken on the same CPU that lockdep is unaware of.
+	 */
+	raw_spin_lock_irqsave_nested(&rp->lock, flags, 1);
 	if (!hlist_empty(&rp->free_instances)) {
 		ri = hlist_entry(rp->free_instances.first,
 				struct kretprobe_instance, hlist);
@@ -2039,7 +2056,7 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 		ri->task = current;
 
 		if (rp->entry_handler && rp->entry_handler(ri, regs)) {
-			raw_spin_lock_irqsave(&rp->lock, flags);
+			raw_spin_lock_irqsave_nested(&rp->lock, flags, 1);
 			hlist_add_head(&ri->hlist, &rp->free_instances);
 			raw_spin_unlock_irqrestore(&rp->lock, flags);
 			return 0;
-- 
2.25.4