Xen Security Advisory XSA-274
Linux: Uninitialized state in PV syscall return path
Linux has a `failsafe` callback, invoked by Xen under certain
conditions. Normally in this failsafe callback, error_entry is paired
with error_exit; and error_entry uses %ebx to communicate to
error_exit whether to use the user or kernel return path.
Unfortunately, on 64-bit PV Xen on x86, error_exit is called without
error_entry being called first, leaving %ebx with an invalid value.
A rogue user-space program could crash a guest kernel. Privilege
escalation cannot be ruled out.
Only 64-bit x86 PV Linux systems are vulnerable.
All versions of Linux are vulnerable.
Switching to HVM or PVH guests will mitigate this issue.
This issue was discovered by M. Vefa Bicakci, and recognized as a
security issue by Andy Lutorminski.
Applying the appropriate attached patch resolves this issue.
NB this patch has not been accepted into Linux upstream yet. An
updated advisory will be sent if the fix upstreamed looks
xsa274-linux-4.17.patch Linux 4.17
$ sha256sum xsa274*
NOTE ON THE LACK OF EMBARGO
The patch for this issue was published on linux-kernel without being
first reported to the XenProject Security Team.
Name: M. Vefa Bicakci, the Xen project
Upstream: Andy Lutorminski
Created xen tracking bugs for this issue:
Affects: fedora-all [bug 1608560]
This bug is now closed. Further updates for individual products will be reflected on the CVE page(s):
This issue only affects guests running as Xen paravirtualized (PV) guests. Starting with Red Hat Enterprise Linux 7 onwards running Red Hat Enterprise Linux installations as Xen PV guests is not supported.