Bug 1679327

Summary: xen: xsa293: PV kernel context switch corruption
Product: [Other] Security Response Reporter: Pedro Sampaio <psampaio>
Component: vulnerabilityAssignee: Red Hat Product Security <security-response-team>
Status: CLOSED NOTABUG QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: ailan, drjones, imammedo, jforbes, knoel, m.a.young, mrezanin, pbonzini, rkrcmar, robinlee.sysu, security-response-team, vkuznets, xen-maint
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
Fixed In Version: Doc Type: If docs needed, set a value
Doc Text:
Story Points: ---
Clone Of: Environment:
Last Closed: 2019-06-10 10:48:29 UTC Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Bug Depends On: 1685577    
Bug Blocks:    

Description Pedro Sampaio 2019-02-20 21:56:06 UTC
On hardware supporting the fsgsbase feature, 64bit PV guests can set and
clear the applicable control bit in its virtualised %cr4, but the
feature remains fully active in hardware. Therefore, the associated
instructions are actually usable.

Linux, which does not currently support this feature, has various
optimisations in its context switch path which justifiably assume that
userspace can't actually make changes without a system call.

Xen's behaviour of having this feature active behind the guest kernel's
back undermines the correctness of any context switch logic which
depends on the feature being disabled.

Userspace can therefore corrupt fsbase or gsbase (commonly used for
Thread Local Storage) in the next thread to be scheduled on the
current vcpu.

Comment 1 Andrej Nemec 2019-03-05 14:21:27 UTC


Comment 2 Andrej Nemec 2019-03-05 14:26:32 UTC
Created xen tracking bugs for this issue:

Affects: fedora-all [bug 1685577]