Patch "KVM: MTRR: remove MSR 0x2f8" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

    KVM: MTRR: remove MSR 0x2f8

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     kvm-mtrr-remove-msr-0x2f8.patch

and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Honig <[email protected]>
Date: Tue, 17 May 2016 17:41:47 +0200
Subject: KVM: MTRR: remove MSR 0x2f8
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From: Andy Honig <[email protected]>

commit 9842df62004f366b9fed2423e24df10542ee0dc5 upstream.

MSR 0x2f8 accessed the 124th Variable Range MTRR ever since MTRR support was introduced by 9ba075a664df ("KVM: MTRR support").

0x2f8 became harmful when 910a6aae4e2e ("KVM: MTRR: exactly define the size of variable MTRRs") shrinked the array of VR MTRRs from 256 to 8, which made access to index 124 out of bounds. The surrounding code only WARNs in this situation, thus the guest gained a limited read/write access to struct kvm_arch_vcpu.

0x2f8 is not a valid VR MTRR MSR, because KVM has/advertises only 16 VR MTRR MSRs, 0x200-0x20f. Every VR MTRR is set up using two MSRs, 0x2f8 was treated as a PHYSBASE and 0x2f9 would be its PHYSMASK, but 0x2f9 was not implemented in KVM, therefore 0x2f8 could never do anything useful and getting rid of it is safe.

This fixes CVE-2016-3713.

---
 arch/x86/kvm/mtrr.c |    2 --
 1 file changed, 2 deletions(-)

@@ -44,8 +44,6 @@ static bool msr_mtrr_valid(unsigned msr)
 	case MSR_MTRRdefType:
 	case MSR_IA32_CR_PAT:
 		return true;
-	case 0x2f8:
-		return true;
 	}
 	return false;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/kvm-mtrr-remove-msr-0x2f8.patch