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

Vulnerability Description

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 mtrr_save_state() 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.

References

• https://git.kernel.org/stable/c/34f36e6ee5bd7eff8b2adcd9fcaef369f752d82e
• https://git.kernel.org/stable/c/06c1de44d378ec5439db17bf476507d68589bfe9
• https://git.kernel.org/stable/c/450b6b22acdaac67a18eaf5ed498421ffcf10051
• https://git.kernel.org/stable/c/ca7d00c5656d1791e28369919e3e10febe9c3b16
• https://git.kernel.org/stable/c/8aa79dfb216b865e96ff890bc4ea71650f9bc8d7
• https://git.kernel.org/stable/c/8a90d3fc7c24608548d3a750671f9dac21d1a462
• https://git.kernel.org/stable/c/388f1c954019f253a8383f7eb733f38d541e10b6
• https://git.kernel.org/stable/c/919f18f961c03d6694aa726c514184f2311a4614