Linux x86 MTRR Fix: Prevent #GP Exception on CPUs without Fixed MTRR Support

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. However, the function does not check this capability bit before accessing the fixed MTRR MSRs. - Impact: On CPUs that do not support the fixed MTRR capability, this causes a #GP (General Protection Fault) exception. While the #GP itself is harmless (as RDMSR faults are handled by WARN_ON()), it still triggers a WARN_ON(). - Fix: Add the missing capability check to prevent this issue. 2. Fix Code: - Modified the function to include a check for fixed MTRR capability. - Code snippet: 3. Patch Information: - Patch ID: 2b1f6278d77c - Patch Title: x86: Save the MTRRs of the BSP before booting an AP - Author: Andi Kleen - Signed-off-by: Thomas Gleixner - CC: stable@vger.kernel.org - Link: https://lore.kernel.org/all/20240808000244.946864-1-ak@linux.intel.com 4. Diffstat: - File Changes: 1 file changed, 1 insertion(+), 1 deletion(-) - Diff Content: This information indicates that the vulnerability was caused by the absence of a capability check for fixed MTRRs, which has now been fixed and submitted as a patch to the Linux kernel.

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Linux x86 MTRR Fix: Prevent #GP Exception on CPUs without Fixed MTRR Support