cifs: prevent NULL pointer dereference in UTF16 conversion

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: cifs: prevent NULL pointer dereference in UTF16 conversion There can be a NULL pointer dereference bug here. NULL is passed to __cifs_sfu_make_node without checks, which passes it unchecked to cifs_strndup_to_utf16, which in turn passes it to cifs_local_to_utf16_bytes where '*from' is dereferenced, causing a crash. This patch adds a check for NULL 'src' in cifs_strndup_to_utf16 and returns NULL early to prevent dereferencing NULL pointer. Found by Linux Verification Center (linuxtesting.org) with SVACE

References

• https://git.kernel.org/stable/c/1f797f062b5cf13a1c2bcc23285361baaa7c9260
• https://git.kernel.org/stable/c/3c26a8d30ed6b53a52a023ec537dc50a6d34a67a
• https://git.kernel.org/stable/c/70bccd9855dae56942f2b18a08ba137bb54093a0