CVE-2020-25596

An issue was discovered in Xen through 4.14.x. x86 PV guest kernels can
experience denial of service via SYSENTER. The SYSENTER instruction leaves
various state sanitization activities to software. One of Xen’s
sanitization paths injects a #GP fault, and incorrectly delivers it twice
to the guest. This causes the guest kernel to observe a kernel-privilege
#GP fault (typically fatal) rather than a user-privilege #GP fault (usually
converted into SIGSEGV/etc.). Malicious or buggy userspace can crash the
guest kernel, resulting in a VM Denial of Service. All versions of Xen from
3.2 onwards are vulnerable. Only x86 systems are vulnerable. ARM platforms
are not vulnerable. Only x86 systems that support the SYSENTER instruction
in 64bit mode are vulnerable. This is believed to be Intel, Centaur, and
Shanghai CPUs. AMD and Hygon CPUs are not believed to be vulnerable. Only
x86 PV guests can exploit the vulnerability. x86 PVH / HVM guests cannot
exploit the vulnerability.

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