linux-kernel - [PATCH] kprobes: Reduce preempt disable scope in check_kprobe_access

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Message-Id: <20241121-kprobes-preempt-v1-1-fd581ee7fcbb@linutronix.de>
Date: Thu, 21 Nov 2024 08:38:08 +0100
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Naveen N Rao <naveen@...nel.org>, 
 Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Masami Hiramatsu <mhiramat@...nel.org>, 
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
 linux-rt-devel@...ts.linux.dev, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH] kprobes: Reduce preempt disable scope in
 check_kprobe_access_safe()

Commit a189d0350f387 ("kprobes: disable preempt for module_text_address() and kernel_text_address()")
introduced a preempt_disable() region to protect against concurrent
module unloading. However this region also includes the call to
jump_label_text_reserved() which takes a long time;
up to 400us, iterating over approx 6000 jump tables.

The scope protected by preempt_disable() is largen than necessary.
core_kernel_text() does not need to be protected as it does not interact
with module code at all.
Only the scope from __module_text_address() to try_module_get() needs to
be protected.
By limiting the critical section to __module_text_address() and
try_module_get() the function responsible for the latency spike remains
preemptible.

This works fine even when !CONFIG_MODULES as in that case
try_module_get() will always return true and that block can be optimized
away.

Limit the critical section to __module_text_address() and
try_module_get(). Use guard(preempt)() for easier error handling.

While at it also remove a spurious *probed_mod = NULL in an error
path. On errors the output parameter is never inspected by the caller.
Some error paths were clearing the parameters, some didn't.
Align them for clarity.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 kernel/kprobes.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index da59c68df8412c4662d39d3e286d516e8cee9a69..369020170e832a34ea9c05eda6693bded02ec505 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -39,6 +39,7 @@
 #include <linux/static_call.h>
 #include <linux/perf_event.h>
 #include <linux/execmem.h>
+#include <linux/cleanup.h>

 #include <asm/sections.h>
 #include <asm/cacheflush.h>
@@ -1570,16 +1571,25 @@ static int check_kprobe_address_safe(struct kprobe *p,
 	if (ret)
 		return ret;
 	jump_label_lock();
-	preempt_disable();

 	/* Ensure the address is in a text area, and find a module if exists. */
 	*probed_mod = NULL;
 	if (!core_kernel_text((unsigned long) p->addr)) {
+		guard(preempt)();
 		*probed_mod = __module_text_address((unsigned long) p->addr);
 		if (!(*probed_mod)) {
 			ret = -EINVAL;
 			goto out;
 		}
+
+		/*
+		 * We must hold a refcount of the probed module while updating
+		 * its code to prohibit unexpected unloading.
+		 */
+		if (unlikely(!try_module_get(*probed_mod))) {
+			ret = -ENOENT;
+			goto out;
+		}
 	}
 	/* Ensure it is not in reserved area. */
 	if (in_gate_area_no_mm((unsigned long) p->addr) ||
@@ -1588,21 +1598,13 @@ static int check_kprobe_address_safe(struct kprobe *p,
 	    static_call_text_reserved(p->addr, p->addr) ||
 	    find_bug((unsigned long)p->addr) ||
 	    is_cfi_preamble_symbol((unsigned long)p->addr)) {
+		module_put(*probed_mod);
 		ret = -EINVAL;
 		goto out;
 	}

 	/* Get module refcount and reject __init functions for loaded modules. */
 	if (IS_ENABLED(CONFIG_MODULES) && *probed_mod) {
-		/*
-		 * We must hold a refcount of the probed module while updating
-		 * its code to prohibit unexpected unloading.
-		 */
-		if (unlikely(!try_module_get(*probed_mod))) {
-			ret = -ENOENT;
-			goto out;
-		}
-
 		/*
 		 * If the module freed '.init.text', we couldn't insert
 		 * kprobes in there.
@@ -1610,13 +1612,11 @@ static int check_kprobe_address_safe(struct kprobe *p,
 		if (within_module_init((unsigned long)p->addr, *probed_mod) &&
 		    !module_is_coming(*probed_mod)) {
 			module_put(*probed_mod);
-			*probed_mod = NULL;
 			ret = -ENOENT;
 		}
 	}

 out:
-	preempt_enable();
 	jump_label_unlock();

 	return ret;

---
base-commit: adc218676eef25575469234709c2d87185ca223a
change-id: 20241120-kprobes-preempt-09826f5e5d9c

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@...utronix.de>