tracing/user_events: Prevent same address and bit per process

User processes register an address and bit pair for events. If the same
address and bit pair are registered multiple times in the same process,
it can cause undefined behavior when events are enabled/disabled.
When more than one are used, the bit could be turned off by another
event being disabled, while the original event is still enabled.

Prevent undefined behavior by checking the current mm to see if any
event has already been registered for the address and bit pair. Return
EADDRINUSE back to the user process if it's already being used.

Update ftrace self-test to ensure this occurs properly.

Link: https://lkml.kernel.org/r/20230425225107.8525-4-beaub@linux.microsoft.comSuggested-by: Doug Cook <dcook@linux.microsoft.com>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_user.c

@@ -419,6 +419,21 @@ static int user_event_enabler_write(struct user_event_mm *mm,
 	return 0;
 }
 
+static bool user_event_enabler_exists(struct user_event_mm *mm,
+				      unsigned long uaddr, unsigned char bit)
+{
+	struct user_event_enabler *enabler;
+	struct user_event_enabler *next;
+
+	list_for_each_entry_safe(enabler, next, &mm->enablers, link) {
+		if (enabler->addr == uaddr &&
+		    (enabler->values & ENABLE_VAL_BIT_MASK) == bit)
+			return true;
+	}
+
+	return false;
+}
+
 static void user_event_enabler_update(struct user_event *user)
 {
 	struct user_event_enabler *enabler;
@@ -657,6 +672,22 @@ void user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)
 	user_event_mm_remove(t);
 }
 
+static bool current_user_event_enabler_exists(unsigned long uaddr,
+					      unsigned char bit)
+{
+	struct user_event_mm *user_mm = current_user_event_mm();
+	bool exists;
+
+	if (!user_mm)
+		return false;
+
+	exists = user_event_enabler_exists(user_mm, uaddr, bit);
+
+	user_event_mm_put(user_mm);
+
+	return exists;
+}
+
 static struct user_event_enabler
 *user_event_enabler_create(struct user_reg *reg, struct user_event *user,
 			   int *write_result)
@@ -2048,6 +2079,16 @@ static long user_events_ioctl_reg(struct user_event_file_info *info,
 	if (ret)
 		return ret;
 
+	/*
+	 * Prevent users from using the same address and bit multiple times
+	 * within the same mm address space. This can cause unexpected behavior
+	 * for user processes that is far easier to debug if this is explictly
+	 * an error upon registering.
+	 */
+	if (current_user_event_enabler_exists((unsigned long)reg.enable_addr,
+					      reg.enable_bit))
+		return -EADDRINUSE;
+
 	name = strndup_user((const char __user *)(uintptr_t)reg.name_args,
 			    MAX_EVENT_DESC);
 

tools/testing/selftests/user_events/ftrace_test.c

@@ -219,7 +219,12 @@ TEST_F(user, register_events) {
 	ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
 	ASSERT_EQ(0, reg.write_index);
 
-	/* Multiple registers should result in same index */
+	/* Multiple registers to the same addr + bit should fail */
+	ASSERT_EQ(-1, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
+	ASSERT_EQ(EADDRINUSE, errno);
+
+	/* Multiple registers to same name should result in same index */
+	reg.enable_bit = 30;
 	ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
 	ASSERT_EQ(0, reg.write_index);
 
@@ -242,6 +247,8 @@ TEST_F(user, register_events) {
 
 	/* Unregister */
 	ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSUNREG, &unreg));
+	unreg.disable_bit = 30;
+	ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSUNREG, &unreg));
 
 	/* Delete should work only after close and unregister */
 	close(self->data_fd);