linux-kernel - [PATCH 2/3] tracing/user_events: Ensure bit is cleared on unregister

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Message-Id: <20230411211709.15018-3-beaub@linux.microsoft.com>
Date:   Tue, 11 Apr 2023 14:17:08 -0700
From:   Beau Belgrave <beaub@...ux.microsoft.com>
To:     rostedt@...dmis.org, mhiramat@...nel.org,
        mathieu.desnoyers@...icios.com, dcook@...ux.microsoft.com,
        alanau@...ux.microsoft.com
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: [PATCH 2/3] tracing/user_events: Ensure bit is cleared on unregister

If an event is enabled and a user process unregisters user_events, the
bit is left set. Fix this by always clearing the bit in the user process
if unregister is successful.

Update abi self-test to ensure this occurs properly.

Suggested-by: Doug Cook <dcook@...ux.microsoft.com>
Signed-off-by: Beau Belgrave <beaub@...ux.microsoft.com>
---
 kernel/trace/trace_events_user.c              | 34 +++++++++++++++++++
 .../testing/selftests/user_events/abi_test.c  |  9 +++--
 2 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index e7dff24aa724..eb195d697177 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -2146,6 +2146,35 @@ static long user_unreg_get(struct user_unreg __user *ureg,
 	return ret;
 }
 
+static int user_event_mm_clear_bit(struct user_event_mm *user_mm,
+				   unsigned long uaddr, unsigned char bit)
+{
+	struct user_event_enabler enabler;
+	int result;
+
+	memset(&enabler, 0, sizeof(enabler));
+	enabler.addr = uaddr;
+	enabler.values = bit;
+retry:
+	/* Prevents state changes from racing with new enablers */
+	mutex_lock(&event_mutex);
+
+	/* Force the bit to be cleared, since no event is attached */
+	mmap_read_lock(user_mm->mm);
+	result = user_event_enabler_write(user_mm, &enabler, false);
+	mmap_read_unlock(user_mm->mm);
+
+	mutex_unlock(&event_mutex);
+
+	if (result) {
+		/* Attempt to fault-in and retry if it worked */
+		if (!user_event_mm_fault_in(user_mm, uaddr))
+			goto retry;
+	}
+
+	return result;
+}
+
 /*
  * Unregisters an enablement address/bit within a task/user mm.
  */
@@ -2190,6 +2219,11 @@ static long user_events_ioctl_unreg(unsigned long uarg)
 
 	mutex_unlock(&event_mutex);
 
+	/* Ensure bit is now cleared for user, regardless of event status */
+	if (!ret)
+		ret = user_event_mm_clear_bit(mm, reg.disable_addr,
+					      reg.disable_bit);
+
 	return ret;
 }
 
diff --git a/tools/testing/selftests/user_events/abi_test.c b/tools/testing/selftests/user_events/abi_test.c
index e0323d3777a7..5125c42efe65 100644
--- a/tools/testing/selftests/user_events/abi_test.c
+++ b/tools/testing/selftests/user_events/abi_test.c
@@ -109,13 +109,16 @@ TEST_F(user, enablement) {
 	ASSERT_EQ(0, change_event(false));
 	ASSERT_EQ(0, self->check);
 
-	/* Should not change after disable */
+	/* Ensure kernel clears bit after disable */
 	ASSERT_EQ(0, change_event(true));
 	ASSERT_EQ(1, self->check);
 	ASSERT_EQ(0, reg_disable(&self->check, 0));
+	ASSERT_EQ(0, self->check);
+
+	/* Ensure doesn't change after unreg */
+	ASSERT_EQ(0, change_event(true));
+	ASSERT_EQ(0, self->check);
 	ASSERT_EQ(0, change_event(false));
-	ASSERT_EQ(1, self->check);
-	self->check = 0;
 }
 
 TEST_F(user, bit_sizes) {
-- 
2.25.1