net: dsa: mv88e6xxx: Correct check for empty list

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net: dsa: mv88e6xxx: Correct check for empty list Since commit a3c53be55c95 ("net: dsa: mv88e6xxx: Support multiple MDIO busses") mv88e6xxx_default_mdio_bus() has checked that the return value of list_first_entry() is non-NULL. This appears to be intended to guard against the list chip->mdios being empty. However, it is not the correct check as the implementation of list_first_entry is not designed to return NULL for empty lists. Instead, use list_first_entry_or_null() which does return NULL if the list is empty. Flagged by Smatch. Compile tested only.

References

• https://git.kernel.org/stable/c/47d28dde172696031c880c5778633cdca30394ee
• https://git.kernel.org/stable/c/3bf8d70e1455f87856640c3433b3660a31001618
• https://git.kernel.org/stable/c/2a2fe25a103cef73cde356e6d09da10f607e93f5
• https://git.kernel.org/stable/c/8c2c3cca816d074c75a2801d1ca0dea7b0148114
• https://git.kernel.org/stable/c/aa03f591ef31ba603a4a99d05d25a0f21ab1cd89
• https://git.kernel.org/stable/c/3f25b5f1635449036692a44b771f39f772190c1d
• https://git.kernel.org/stable/c/f75625db838ade28f032dacd0f0c8baca42ecde4
• https://git.kernel.org/stable/c/4c7f3950a9fd53a62b156c0fe7c3a2c43b0ba19b