CVE-2026-98549

tcp: prevent collapsing skbs across boundary in rtx queue

Published Oct 18, 2026 CVSS 3.1 · 9.1 Critical EPSS < 1% KEV · Not listed
Description
In the Linux kernel, the following vulnerability has been resolved:

tcp: prevent collapsing skbs across boundary in rtx queue

tcp_write_collapse_fence() sets TCP_SKB_CB(skb)->eor = 1 on
tcp_write_queue_tail(sk) to prevent skbs queued after a switch to
device encryption from being collapsed into earlier skbs.

The fence is a no-op if all earlier data has already been transmitted
when the switch happens: sk->sk_write_queue is empty. The not yet
acknowledged earlier skbs wait in sk->tcp_rtx_queue with eor 0.

On a subsequent retransmit or SACK shift, tcp_retrans_try_collapse() or
tcp_shift_skb_data() can then merge an skb queued after the switch into
one queued before it.

Both users of the fence are affected:

- psp: devices only encrypt skbs with skb->decrypted set. The merged skb
keeps decrypted = 0 from the earlier skb, so merged data sent after
psp_sock_assoc_set_tx() is retransmitted in cleartext.

- tls device offload: the merged skb straddles the start marker set in
tls_set_device_offload(). The software fallback (fill_sg_in() returns
-EINVAL) and the mlx5, nfp and funeth drivers cannot handle such an
skb and drop it. Every retransmit rebuilds the same skb, so the
connection stalls.

Fix this in two places, for defense in depth:

1. Fall back to tcp_rtx_queue_tail(sk) in tcp_write_collapse_fence()
when tcp_write_queue_tail(sk) is NULL.

2. Check !skb_cmp_decrypted(to, from) in tcp_skb_can_collapse(), as
tcp_skb_can_collapse_rx() does on receive. skb_shift(), which both
collapse paths call, already has a DEBUG_NET_WARN_ON_ONCE() for this
condition.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DLA Debian DLA DLA-4850-1 linux-6.12 security update
Ubuntu USN Ubuntu USN USN-9019-1 Linux kernel (Azure) vulnerabilities
Ubuntu USN Ubuntu USN USN-9020-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9022-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9023-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9024-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9025-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9026-1 Linux kernel (Azure) vulnerabilities
Ubuntu USN Ubuntu USN USN-9039-1 Linux kernel (Azure CVM) vulnerabilities
Ubuntu USN Ubuntu USN USN-9040-1 Linux kernel (Low Latency NVIDIA) vulnerabilities
Ubuntu USN Ubuntu USN USN-9066-1 Linux kernel (HWE) vulnerabilities
Ubuntu USN Ubuntu USN USN-9091-1 Linux kernel (NVIDIA Tegra) vulnerabilities
History
Type Values Removed Values Added
Metrics cvssV3_1

{'score': 4.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:L'}

cvssV3_1

{'score': 9.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H'}

Type Values Removed Values Added
Weaknesses CWE-200
CWE-310
Type Values Removed Values Added
Weaknesses CWE-319
References
Metrics threat_severity

None

cvssV3_1

{'score': 4.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:L'}

threat_severity

Moderate

Type Values Removed Values Added
Weaknesses CWE-200
CWE-310
Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: tcp: prevent collapsing skbs across boundary in rtx queue tcp_write_collapse_fence() sets TCP_SKB_CB(skb)->eor = 1 on tcp_write_queue_tail(sk) to prevent skbs queued after a switch to device encryption from being collapsed into earlier skbs. The fence is a no-op if all earlier data has already been transmitted when the switch happens: sk->sk_write_queue is empty. The not yet acknowledged earlier skbs wait in sk->tcp_rtx_queue with eor 0. On a subsequent retransmit or SACK shift, tcp_retrans_try_collapse() or tcp_shift_skb_data() can then merge an skb queued after the switch into one queued before it. Both users of the fence are affected: - psp: devices only encrypt skbs with skb->decrypted set. The merged skb keeps decrypted = 0 from the earlier skb, so merged data sent after psp_sock_assoc_set_tx() is retransmitted in cleartext. - tls device offload: the merged skb straddles the start marker set in tls_set_device_offload(). The software fallback (fill_sg_in() returns -EINVAL) and the mlx5, nfp and funeth drivers cannot handle such an skb and drop it. Every retransmit rebuilds the same skb, so the connection stalls. Fix this in two places, for defense in depth: 1. Fall back to tcp_rtx_queue_tail(sk) in tcp_write_collapse_fence() when tcp_write_queue_tail(sk) is NULL. 2. Check !skb_cmp_decrypted(to, from) in tcp_skb_can_collapse(), as tcp_skb_can_collapse_rx() does on receive. skb_shift(), which both collapse paths call, already has a DEBUG_NET_WARN_ON_ONCE() for this condition.
Title tcp: prevent collapsing skbs across boundary in rtx queue
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References
Weaknesses

Sources

Status
PUBLISHED
Assigner
Linux
Published
Updated
Oct 30, 2026, 12:38 PM
Reserved
Sep 25, 2026, 10:25 AM
Status
Received
Published
Oct 18, 2026, 4:17 PM
Modified
Oct 30, 2026, 1:21 PM
Severity
Moderate
Public date
Oct 18, 2026, 12:00 AM