Bug 1995153 (CVE-2021-29657)

Summary:	CVE-2021-29657 kernel: KVM: double fetch in nested_svm_vmrun can lead to unrestricted MSR access
Product:	[Other] Security Response	Reporter:	Pedro Sampaio <psampaio>
Component:	vulnerability	Assignee:	Red Hat Product Security <security-response-team>
Status:	CLOSED ERRATA	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	acaringi, adscvr, airlied, alciregi, bhu, blc, brdeoliv, bskeggs, chwhite, crwood, dhoward, dvlasenk, fhrbata, fpacheco, hdegoede, hkrzesin, jarod, jarodwilson, jeremy, jforbes, jlelli, jonathan, josef, jshortt, jstancek, jwboyer, kcarcia, kernel-maint, kernel-mgr, kpatch-maint, lgoncalv, linville, masami256, mchehab, mlangsdo, nmurray, ptalbert, qzhao, rhandlin, rvrbovsk, steved, walters, williams
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:	kernel 5.12	Doc Type:	If docs needed, set a value
Doc Text:	A flaw was found in the Linux kernel. A KVM guest on AMD can launch a nested guest without the Intercept VMRUN control bit by exploiting a TOCTOU vulnerability in nested_svm_vmrun. A malicious guest could use this flaw to gain unrestricted access to host MSRs, possibly leading to guest-to-host escape scenario.	Story Points:	---
Clone Of:		Environment:
Last Closed:	2021-12-15 11:28:40 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	1945742, 1997736, 1997737
Bug Blocks:	1995155

Description Pedro Sampaio 2021-08-18 14:28:43 UTC

A KVM guest on AMD can launch a L2 guest without the Intercept VMRUN control bit by exploiting a TOCTOU vulnerability in nested_svm_vmrun. Executing VMRUN from the L2 guest, will then trigger a second call to nested_svm_vmrun and corrupt svm->nested.hsave with data copied out of the L2 vmcb. For kernel versions that include the commit "2fcf4876: KVM: nSVM: implement on demand allocation of the nested state" (>=5.10), the guest can free the MSR permission bit in svm->nested.msrpm, while it's still in use and gain unrestricted access to host MSRs.

Upstream fix:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a58d9166a756a0f4a6618e4f593232593d6df134

Reference:
https://bugs.chromium.org/p/project-zero/issues/detail?id=2177

Comment 4 juneau 2021-08-26 13:40:50 UTC

Setting Hosted OCP 'notaffected' as KVM is not employed in this environment.