CVE-2024-44948

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

x86/mtrr: Check if fixed MTRRs exist before saving them

MTRRs have an obsolete fixed variant for fine grained caching control of the 640K-1MB region that uses separate MSRs. This fixed variant has a separate capability bit in the MTRR capability MSR.

So far all x86 CPUs which support MTRR have this separate bit set, so it went unnoticed that mtrrsavestate() does not check the capability bit before accessing the fixed MTRR MSRs.

Though on a CPU that does not support the fixed MTRR capability this results in a #GP. The #GP itself is harmless because the RDMSR fault is handled gracefully, but results in a WARN_ON().

Add the missing capability check to prevent this.