[PATCH 1/3] tracing/user_events: Ensure write index cannot be negative

[Beau Belgrave <beaub@linux.microsoft.com>]

The write index indicates which event the data is for and accesses a
per-file array. The index is passed by user processes during write()
calls as the first 4 bytes. Ensure that it cannot be negative by
returning -EINVAL to prevent out of bounds accesses.

Update ftrace self-test to ensure this occurs properly.

Fixes: 7f5a08c79df3 ("user_events: Add minimal support for trace_event into ftrace")
Reported-by: Doug Cook <dcook@linux.microsoft.com>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
---
 kernel/trace/trace_events_user.c                  | 3 +++
 tools/testing/selftests/user_events/ftrace_test.c | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index cc8c6d8b69b5..e7dff24aa724 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -1821,6 +1821,9 @@ static ssize_t user_events_write_core(struct file *file, struct iov_iter *i)
 	if (unlikely(copy_from_iter(&idx, sizeof(idx), i) != sizeof(idx)))
 		return -EFAULT;
 
+	if (idx < 0)
+		return -EINVAL;
+
 	rcu_read_lock_sched();
 
 	refs = rcu_dereference_sched(info->refs);
diff --git a/tools/testing/selftests/user_events/ftrace_test.c b/tools/testing/selftests/user_events/ftrace_test.c
index aceafacfb126..91272f9d6fce 100644
--- a/tools/testing/selftests/user_events/ftrace_test.c
+++ b/tools/testing/selftests/user_events/ftrace_test.c
@@ -296,6 +296,11 @@ TEST_F(user, write_events) {
 	ASSERT_NE(-1, writev(self->data_fd, (const struct iovec *)io, 3));
 	after = trace_bytes();
 	ASSERT_GT(after, before);
+
+	/* Negative index should fail with EINVAL */
+	reg.write_index = -1;
+	ASSERT_EQ(-1, writev(self->data_fd, (const struct iovec *)io, 3));
+	ASSERT_EQ(EINVAL, errno);
 }
 
 TEST_F(user, write_fault) {
-- 
2.25.1