Bug 2535099 (CVE-2026-89793)

Summary:	CVE-2026-89793 kernel: ublk: clear VM_MAYWRITE on read-only ublk char device mmap
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in the Linux kernel's ublk subsystem. An unprivileged userspace daemon can exploit this by mapping a read-only ublk character device's command buffer, then elevating its permissions to write access. This allows the daemon to corrupt critical kernel-written data within the command buffer, potentially leading to system instability or unauthorized operations.	Story Points:	---
Clone Of:		Environment:
Last Closed:		Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-16 10:08:55 UTC

In the Linux kernel, the following vulnerability has been resolved:

ublk: clear VM_MAYWRITE on read-only ublk char device mmap

ublk_ch_mmap() rejects mmap requests with VM_WRITE set, but never
clears VM_MAYWRITE on the resulting read-only mapping. This allows
a userspace daemon to mmap the per-queue command buffer PROT_READ,
then upgrade it to PROT_WRITE via mprotect(), since VM_MAYWRITE was
never cleared.

The command buffer holds struct ublksrv_io_desc entries that are
kernel-written ABI; a writable mapping lets an unprivileged daemon
process corrupt fields such as addr, op_flags, nr_sectors, and
start_sector.

Same bug class as the drm/panthor and drm/vc4 VM_MAYWRITE fixes, and
the 2026-08-13 ptp/vmclock fix (a5edadbae57e).

Verified via mprotect() PoC: before the fix, a PROT_READ mapping can
be upgraded to PROT_READ|PROT_WRITE and a write into the command
buffer corrupts io_desc fields (confirmed under KASAN). After the
fix, mprotect() returns -EACCES.

Comment 1 Keith Grant 2026-09-21 21:01:50 UTC

Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89793.mbox