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computer vulnerability bulletin CVE-2016-3158 CVE-2016-3159

Xen: information disclosure during arithmetic operations

Synthesis of the vulnerability

When Xen is installed on AMD processors Family 15 (or greater), an attacker located in a guest system can obtain information during arithmetic operations.
Vulnerable products: XenServer, Debian, Fedora, openSUSE, openSUSE Leap, SUSE Linux Enterprise Desktop, SLES, Xen.
Severity of this weakness: 1/4.
Consequences of an attack: data reading.
Hacker's origin: user shell.
Number of vulnerabilities in this bulletin: 2.
Creation date: 29/03/2016.
Références of this bulletin: CERTFR-2016-AVI-133, CTX209443, CVE-2016-3158, CVE-2016-3159, DLA-571-1, DSA-3554-1, FEDORA-2016-5f196e4e4a, FEDORA-2016-e5432ca977, openSUSE-SU-2016:2494-1, openSUSE-SU-2016:2497-1, SUSE-SU-2016:2093-1, SUSE-SU-2016:2100-1, SUSE-SU-2016:2528-1, SUSE-SU-2016:2533-1, SUSE-SU-2016:2725-1, VIGILANCE-VUL-19248, XSA-172.

Description of the vulnerability

The Floating Point Unit (FPU, x87) is used during floating point arithmetic operations. The FPU contains 3 debug registers: FOP, FIP and FDP. The FSAVE/FSTOR or FXSAVE/FXSTOR instructions save and restore these registers. The support is enabled via the "xsave" option.

The xrstor() function of the xen/arch/x86/i387.c file does not manage the case of AMD processor, which require these registers to be erased, even if FSW.ESis used (to fix VIGILANCE-VUL-12901). So, a guest system can access to the content of the FOP, FIP and FDP registers of another domain.

When Xen is installed on AMD processors Family 15 (or greater), an attacker located in a guest system can therefore obtain information during arithmetic operations.
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