ALSA: control: Validate buf_len before strnlen() in snd_ctl_elem_init_enum_names()

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: 8d44816 ("ALSA: control: add support for ENUMERATED user space controls")
Cc: [email protected]
Signed-off-by: Ziqing Chen <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

Author: chenziqing-cmyk

sound/core/control.c

@@ -1648,6 +1648,10 @@ 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);