Bug 2547433 (CVE-2026-98374)

Summary:	CVE-2026-98374 kernel: tcp: fix use-after-free of retransmit_skb_hint in tcp_send_synack()
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 TCP networking subsystem. When replacing packet buffers during connection establishment, the kernel frees memory without updating an internal retransmission pointer, resulting in a use-after-free vulnerability. An unprivileged local user can exploit this condition using TCP Fast Open (TFO) sockets and crafted network responses. Successful exploitation allows an attacker to crash the system, resulting in a Denial of Service (DoS) or unexpected kernel memory corruption.	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 2026-10-07 13:22:15 UTC

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

tcp: fix use-after-free of retransmit_skb_hint in tcp_send_synack()

When tcp_send_synack() replaces the cloned SYN skb at the head of the
retransmit queue with a copy, it frees the original with
tcp_rtx_queue_unlink_and_free() and only repairs tp->highest_sack.
tp->retransmit_skb_hint keeps pointing at the freed
skbuff_fclone_cache object.

The dangling hint is read in tcp_verify_retransmit_hint() and used as
the root of the rbtree walk in tcp_xmit_retransmit_queue().  An
unprivileged TFO client (sendmsg(MSG_FASTOPEN)) can arm the hint with
an attacker-supplied ICMP fragmentation-needed message, after which a
simultaneous open frees the armed SYN skb:

  BUG: KASAN: slab-use-after-free in tcp_mark_skb_lost (net/ipv4/tcp_input.c:1316)
  Read of size 4 at addr ffff88800604d928 by task swapper/1/0
  Call Trace:
   tcp_mark_skb_lost (net/ipv4/tcp_input.c:1316)
   tcp_simple_retransmit (net/ipv4/tcp_input.c:3158)
   tcp_v4_err (net/ipv4/tcp_ipv4.c:587)

Sync the hint to the copy.

Comment 1 gkamathe 2026-10-09 10:28:27 UTC

Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-98374.mbox