CVE-2024-56685: ASoC: mediatek: Check num_codecs is not zero to avoid panic during probe

In the Linux kernel, the following vulnerability has been resolved:

ASoC: mediatek: Check numcodecs is not zero to avoid panic during probe

Following commit 13f58267cda3 ("ASoC: soc.h: don't create dummy Component via COMPDUMMY()"), COMPDUMMY() became an array with zero length, and only gets populated with the dummy struct after the card is registered. Since the sound card driver's probe happens before the card registration, accessing any of the members of a dummy component during probe will result in undefined behavior.

This can be observed in the mt8188 and mt8195 machine sound drivers. By omitting a dai link subnode in the sound card's node in the Devicetree, the default uninitialized dummy codec is used, and when its dainame pointer gets passed to strcmp() it results in a null pointer dereference and a kernel panic.

In addition to that, setcardcodecinfo() in the generic helpers file, mtk-soundcard-driver.c, will populate a dai link with a dummy codec when a dai link node is present in DT but with no codec property.

The result is that at probe time, a dummy codec can either be uninitialized with numcodecs = 0, or be an initialized dummy codec, with numcodecs = 1 and dainame = "snd-soc-dummy-dai". In order to accommodate for both situations, check that numcodecs is not zero before accessing the codecs' fields but still check for the codec's dai name against "snd-soc-dummy-dai" as needed.

While at it, also drop the check that dainame is not null in the mt8192 driver, introduced in commit 4d4e1b6319e5 ("ASoC: mediatek: mt8192: Check existence of dainame before dereferencing"), as it is actually redundant given the preceding numcodecs != 0 check.