Linux x86 MTRR #GP Exception Fix

From this webpage screenshot, we can extract the following key information about the vulnerability: 1. Vulnerability Description: - Title: x86/mtrr: Check if fixed MTRRs exist before saving them - Description: MTRRs (Memory Type Range Registers) have an obsolete fixed variant used for fine-grained cache control over the 640K–1MB region, which uses separate MSRs (Model-Specific Registers). This fixed variant has a dedicated capability bit in the MTRR capability MSR. - Issue: In all x86 CPUs that support MTRR, this dedicated capability bit is set. As a result, the function accesses the fixed MTRR MSRs without checking this capability bit. - Impact: On CPUs that do not support the fixed MTRR capability, this triggers a #GP (General Protection Fault) exception. While the #GP itself is harmless—since RDMSR faults are handled appropriately—it still causes a WARN_ON(). 2. Fix: - Patch: The patch resolves this issue by ensuring the fixed MTRR capability bit is checked before saving MTRRs. - Author: Andi Kleen - Signed-off-by: Andi Kleen and Thomas Gleixner - Contact: stable@vger.kernel.org - Link: https://lore.kernel.org/all/20240808000244.946864-1-ak@linux.intel.com 3. Code Changes: - File: - Changes: Added code in the function to check the fixed MTRR capability bit before proceeding. 4. Patch Status: - Status: Fixed; the patch has been applied to the kernel. This information indicates that the vulnerability stemmed from insufficient MTRR capability checking, and the fix was implemented by adding a capability check for fixed MTRRs before saving their state.

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Linux x86 MTRR #GP Exception Fix