Bug 1905497 (CVE-2020-29534)

Summary:	CVE-2020-29534 kernel: io_uring takes a non-refcounted reference to the files_struct of the process that submitted a request, causing execve() to incorrectly optimize unshare_fd() allowing for privileges escalation
Product:	[Other] Security Response	Reporter:	Marian Rehak <mrehak>
Component:	vulnerability	Assignee:	Red Hat Product Security <security-response-team>
Status:	CLOSED NOTABUG	QA Contact:
Severity:	high	Docs Contact:
Priority:	high
Version:	unspecified	CC:	acaringi, adscvr, airlied, bhu, blc, bmasney, brdeoliv, bskeggs, dhoward, dvlasenk, fhrbata, hdegoede, hkrzesin, itamar, jarodwilson, jeremy, jforbes, jglisse, jlelli, jonathan, josef, jshortt, jstancek, jwboyer, kcarcia, kernel-maint, kernel-mgr, lgoncalv, linville, masami256, mchehab, mlangsdo, nmurray, ptalbert, qzhao, rvrbovsk, steved, walters, williams
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2020-12-08 15:41:47 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:	1905498, 1906390
Bug Blocks:	1905499

Description Marian Rehak 2020-12-08 13:58:52 UTC

io_uring takes a non-refcounted reference to the files_struct of the process that submitted a request (relying on ->flush() for being notified before the files_struct can go away). Unfortunately, unshare_fd(), which is used by bprm_execve() via unshare_files(), doesn't know about that, and assumes that if the files_struct's refcount is 1, it is okay to keep using the old files_struct. Therefore, an attacker can cause io_uring to fiddle around in the file descriptor table of a privileged process, and in particular steal file descriptors using IORING_OP_FILES_UPDATE.

References:

https://bugs.chromium.org/p/project-zero/issues/detail?id=2089
https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.9.3
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0f2122045b946241a9e549c2a76cea54fa58a7ff

Comment 1 Marian Rehak 2020-12-08 13:59:31 UTC

Created kernel tracking bugs for this issue:

Affects: fedora-all [bug 1905498]

Comment 2 Justin M. Forbes 2020-12-08 15:13:32 UTC

This was fixed for Fedora with the 5.8.18 stable kernel updates.