CVE-2024-56700: 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 fmcsendcmd()

Atomicity violation occurs when the fmcsendcmd() function is executed simultaneously with the modification of the fmdev->respskb value. Consider a scenario where, after passing the validity check within the function, a non-null fmdev->respskb variable is assigned a null value. This results in an invalid fmdev->respskb variable passing the validity check. As seen in the later part of the function, skb = fmdev->respskb; when the invalid fmdev->respskb passes the check, a null pointer dereference error may occur at line 478, evthdr = (void )skb->data;

To address this issue, it is recommended to include the validity check of fmdev->respskb within the locked section of the function. This modification ensures that the value of fmdev->respskb 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.