Bug 2555258 (CVE-2026-98839)

Summary:	CVE-2026-98839 kernel: vt: selection: Fix unsigned underflow and slab-out-of-bounds read in paste_selection()
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 virtual terminal selection subsystem. When pasting console selections, a race condition can occur if another process concurrently replaces the selection buffer with shorter content while the operation is temporarily paused. This situation causes an integer underflow that leads to an out-of-bounds memory read. A local user can exploit this vulnerability to cause a system crash, resulting in a Denial of Service (DoS), or potentially expose sensitive kernel memory.	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 2026-10-18 16:05:13 UTC

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

vt: selection: Fix unsigned underflow and slab-out-of-bounds read in paste_selection()

In paste_selection(), the loop copies min_t(unsigned int,
vc_sel.buf_len - pasted,tty->receive_room) bytes per iteration into
tty_ldisc_receive_buf() and increments pasted += count.

Because the selection mutex (vc_sel.lock) is dropped inside the loop
Whenever the line discipline buffer fills up and paste_selection()
sleeps on tty->write_wait, a concurrent TIOCLINUX (TIOCL_SETSEL) ioctl
can replace vc_sel.buffer with a shorter selection and reduce
vc_sel.buf_len below pasted.

When paste_selection() resumes, vc_sel.buf_len - pasted underflows as an
unsigned integer to a large positive value, causing
tty_ldisc_receive_buf(ld, vc_sel.buffer + pasted, NULL, count) to read
up to 4094 bytes out-of-bounds past the newly allocated vc_sel.buffer.

Fix this by terminating the loop when pasted >= vc_sel.buf_len.

Kernel stack trace (Linux 7.3.0-rc3):
 ==================================================================
 BUG: KASAN: slab-out-of-bounds in n_tty_receive_buf_common+0xa01/0x1650
 Read of size 4094 at addr ffff888101c58010 by task kworker/u17:1/65
 Workqueue: events_unbound flush_to_ldisc
 Call Trace:
  <TASK>
  dump_stack_lvl+0x70/0xa0
  print_report+0x153/0x4c6
  kasan_report+0xf1/0x120
  kasan_check_range+0x11c/0x200
  __asan_memcpy+0x29/0x70
  n_tty_receive_buf_common+0xa01/0x1650
  tty_ldisc_receive_buf+0x66/0x110
  tty_port_default_receive_buf+0x6b/0xb0
  flush_to_ldisc+0x1b4/0x410
  process_one_work+0x6ff/0x1110
  worker_thread+0x4a8/0xb70
  kthread+0x307/0x3e0
  ret_from_fork+0x3ed/0x680
  </TASK>
 ==================================================================

Comment 1 Mauro Matteo Cascella 2026-10-23 21:33:48 UTC

Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-98839.mbox