media: wl128x: Fix atomicity violation in fmc_send_cmd()

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

media: wl128x: Fix atomicity violation in fmc_send_cmd()

Atomicity violation occurs when the fmc_send_cmd() function is executed

simultaneously with the modification of the fmdev->resp_skb value.

Consider a scenario where, after passing the validity check within the

function, a non-null fmdev->resp_skb variable is assigned a null value.

This results in an invalid fmdev->resp_skb variable passing the validity

check. As seen in the later part of the function, skb = fmdev->resp_skb;

when the invalid fmdev->resp_skb passes the check, a null pointer

dereference error may occur at line 478, evt_hdr = (void *)skb->data;

To address this issue, it is recommended to include the validity check of

fmdev->resp_skb within the locked section of the function. This

modification ensures that the value of fmdev->resp_skb does not change

during the validation process, thereby maintaining its validity.

This possible bug is found by an experimental static analysis tool

developed by our team. This tool analyzes the locking APIs

to extract function pairs that can be concurrently executed, and then

analyzes the instructions in the paired functions to identify possible

concurrency bugs including data races and atomicity violations.