linux-kernel - [71/75] KVM: x86: fix for buffer overflow in handling of MSR_KVM_SYSTEM

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Date:	Mon, 22 Apr 2013 15:25:59 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Andy Honig" <ahonig@...gle.com>,
	"Marcelo Tosatti" <mtosatti@...hat.com>
Subject: [71/75] KVM: x86: fix for buffer overflow in handling of
 MSR_KVM_SYSTEM_TIME (CVE-2013-1796)

3.2.44-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Honig <ahonig@...gle.com>

commit c300aa64ddf57d9c5d9c898a64b36877345dd4a9 upstream.

If the guest sets the GPA of the time_page so that the request to update the
time straddles a page then KVM will write onto an incorrect page.  The
write is done byusing kmap atomic to get a pointer to the page for the time
structure and then performing a memcpy to that page starting at an offset
that the guest controls.  Well behaved guests always provide a 32-byte aligned
address, however a malicious guest could use this to corrupt host kernel
memory.

Tested: Tested against kvmclock unit test.

Signed-off-by: Andrew Honig <ahonig@...gle.com>
Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kvm/x86.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1603,6 +1603,11 @@ int kvm_set_msr_common(struct kvm_vcpu *
 		/* ...but clean it before doing the actual write */
 		vcpu->arch.time_offset = data & ~(PAGE_MASK | 1);
 
+		/* Check that the address is 32-byte aligned. */
+		if (vcpu->arch.time_offset &
+				(sizeof(struct pvclock_vcpu_time_info) - 1))
+			break;
+
 		vcpu->arch.time_page =
 				gfn_to_page(vcpu->kvm, data >> PAGE_SHIFT);
 

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