[RESEND PATCH] ALSA: control: Validate buf_len before strnlen() in snd_ctl_elem_init_enum_names()

[Ziqing Chen <chzq96@gmail.com>]

snd_ctl_elem_init_enum_names() advances pointer p through the names
buffer while decrementing buf_len. If buf_len reaches zero but items
remain, the next iteration calls strnlen(p, 0).

While strnlen(p, 0) returns 0 and would hit the existing name_len == 0
error path, CONFIG_FORTIFY_SOURCE's fortified strnlen() first checks
maxlen against __builtin_dynamic_object_size(). When Clang loses track
of p's object size inside the loop, this triggers a BRK exception panic
before the return value is examined.

Add a buf_len == 0 guard at the loop entry to prevent calling fortified
strnlen() on an exhausted buffer.

Found by kernel fuzz testing through Xiaomi Smartphone.

Fixes: 8d448162bda5 ("ALSA: control: add support for ENUMERATED user space controls")
Cc: stable@vger.kernel.org
Signed-off-by: Ziqing Chen <chenziqing@xiaomi.com>
---
 sound/core/control.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/core/control.c b/sound/core/control.c
index 0ddade871b52..6ceb5f977fcd 100644
--- a/sound/core/control.c
+++ b/sound/core/control.c
@@ -1574,6 +1574,10 @@ static int snd_ctl_elem_init_enum_names(struct user_element *ue)
 	/* check that there are enough valid names */
 	p = names;
 	for (i = 0; i < ue->info.value.enumerated.items; ++i) {
+		if (buf_len == 0) {
+			kvfree(names);
+			return -EINVAL;
+		}
 		name_len = strnlen(p, buf_len);
 		if (name_len == 0 || name_len >= 64 || name_len == buf_len) {
 			kvfree(names);
-- 
2.52.0