This bug was fixed before the affected kernel was released and,
therefore, it never affected a publicly released kernel.
** Changed in: linux (Ubuntu)
Status: Confirmed => Fix Released
** Information type changed from Private Security to Public Security
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https://bugs.launchpad.net/bugs/1772146
Title:
Xenial 4.4.0-126.152 does not properly virtualize cpuid, resulting in
disabled SSBD
Status in linux package in Ubuntu:
Fix Released
Bug description:
The Xenial (4.4.0-126.152) kernel, which is to be released on 5/21,
does not properly handle the cpuid opcode for KVM guests. This causes
the guest kernel to incorrectly assume that the Speculative Storage
Bypass Disable (SSBD) feature (needed for CVE-2018-3639) is not
supported and, therefore, causes it to be unusable in a KVM guest.
From the host environment (running 4.4.0-126.152):
$ cpuid -1r | grep '^ 0x00000007'
0x00000007 0x00: eax=0x00000000 ebx=0x000037ab ecx=0x00000000
edx=0x9c000000
When the most significant bit (bit 31) of edx is set, that indicates
that SSBD is available.
Running the same command in the guest shows that the bit is not set:
$ cpuid -1r | grep '^ 0x00000007'
0x00000007 0x00: eax=0x00000000 ebx=0x000007ab ecx=0x00000000
edx=0x00000000
Therefore, we see this message in the guest's dmesg output:
$ dmesg | grep Speculative
[ 0.008000] Speculative Store Bypass: Vulnerable
We also see this in the guest:
$ cat /sys/devices/system/cpu/vulnerabilities/spec_store_bypass
Vulnerable
I don't yet see what's causing this bug when the Xenial 4.4 kernel is
used in the host. It does not occur when the Trusty 3.13, Artful 4.13,
Bionic 4.15 kernels are used in the host.
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