In the Linux kernel, the following vulnerability has been resolved:
x86: fix user address masking non-canonical speculation issue
It turns out that AMD has a "Meltdown Lite(tm)" issue with non-canonical accesses in kernel space. And so using just the high bit to decide whether an access is in user space or kernel space ends up with the good old "leak speculative data" if you have the right gadget using the result:
CVE-2020-12965 “Transient Execution of Non-Canonical Accesses“
Now, the kernel surrounds the access with a STAC/CLAC pair, and those instructions end up serializing execution on older Zen architectures, which closes the speculation window.
But that was true only up until Zen 5, which renames the AC bit [1]. That improves performance of STAC/CLAC a lot, but also means that the speculation window is now open.
Note that this affects not just the new address masking, but also the regular validuseraddress() check used by accessok(), and the asm version of the sign bit check in the getuser() helpers.
It does not affect putuser() or clearuser() variants, since there's no speculative result to be used in a gadget for those operations.