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Vigil@nce - Xen, KVM: infinite loop of x86 Alignment Check Exception

November 2015 by Vigil@nce

This bulletin was written by Vigil@nce : http://vigilance.fr/offer

SYNTHESIS OF THE VULNERABILITY

An attacker, who is administrator in a guest system, can generate
an infinite loop with an Alignment Check Exception on Xen/KVM, in
order to trigger a denial of service on the host system.

Impacted products: XenServer, Debian, Fedora, Linux, Ubuntu, Xen.

Severity: 1/4.

Creation date: 10/11/2015.

DESCRIPTION OF THE VULNERABILITY

On an x86 processor, when an exception occurs, while another
exception is in progress, the second has to be managed
sequentially. The Xen/KVM product implements workarounds to forbid
infinite loops in this case.

However, when an AC (Alignment Check) exception occurs with a
Ring-3 Handler and an unaligned stack pointer, this case is not
managed.

An attacker, who is administrator in a guest system, can therefore
generate an infinite loop with an Alignment Check Exception on
Xen/KVM, in order to trigger a denial of service on the host
system.

ACCESS TO THE COMPLETE VIGIL@NCE BULLETIN

http://vigilance.fr/vulnerability/Xen-KVM-infinite-loop-of-x86-Alignment-Check-Exception-18268


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