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

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commit 919f18f961 upstream.

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.

Fixes: 2b1f6278d7 ("[PATCH] x86: Save the MTRRs of the BSP before booting an AP")
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240808000244.946864-1-ak@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Andi Kleen 2024-08-07 17:02:44 -07:00 committed by Greg Kroah-Hartman
parent d3e4dbc285
commit 06c1de44d3
1 changed files with 1 additions and 1 deletions
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2
arch/x86/kernel/cpu/mtrr/mtrr.c
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@ -817,7 +817,7 @@ void mtrr_save_state(void)
{
int first_cpu;
if (!mtrr_enabled())
if (!mtrr_enabled() || !mtrr_state.have_fixed)
return;
first_cpu = cpumask_first(cpu_online_mask);