CVE-2026-43194
Estado: AnalizadaAlta (7.5)—
In the Linux kernel, the following vulnerability has been resolved:
net: consume xmit errors of GSO frames
udpgro_frglist.sh and udpgro_bench.sh are the flakiest tests currently in NIPA. They fail in the same exact way, TCP GRO test stalls occasionally and the test gets killed after 10min.
These tests use veth to simulate GRO. They attach a trivial ("return XDP_PASS;") XDP program to the veth to force TSO off and NAPI on.
Digging into the failure mode we can see that the connection is completely stuck after a burst of drops.
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The sender's snd_nxt is at sequence number N [1], but the receiver claims to have received (rcv_nxt) up to N + 3 * MSS [2]. Last piece of the puzzle is that senders rtx queue is not empty (let's say the block in the rtx queue is at sequence number N - 4 * MSS [3]).
In this state, sender sends a retransmission from the rtx queue with a single segment, and sequence numbers N-4*MSS:N-3*MSS [3]. Receiver sees it and responds with an ACK all the way up to N + 3 * MSS [2]. But sender will reject this ack as TCP_ACK_UNSENT_DATA because it has no recollection of ever sending data that far out [1]. And we are stuck.
The root cause is the mess of the xmit return codes. veth returns an error when it can't xmit a frame. We end up with a loss event like this:
"x" means packet lost by veth, and "ok" means it went thru. Since veth has TSO disabled in this test it sees individual segments. Segment 1 is on the retransmit queue and will be resent.
So why did the sender not advance snd_nxt even tho it clearly did send up to seg 8? tcp_write_xmit() interprets the return code from the core to mean that data has not been sent at all. Since TCP deals with GSO super frames, not individual segment the crux of the problem is that loss of a single segment can be interpreted as loss of all. TCP only sees the last return code for the last segment of the GSO frame (in <> brackets in the diagram above).
Of course for the problem to occur we need a setup or a device without a Qdisc. Otherwise Qdisc layer disconnects the protocol layer from the device errors completely.
We have multiple ways to fix this.
Similar fix has been proposed by Eric in the past but never committed because original reporter was working with an OOT driver and wasn't providing feedback (see Link).
Detalles técnicos trazas, registros y código del informe original
-------------------------------------------------
| GSO super frame 1 | GSO super frame 2 |
|-----------------------------------------------|
| seg | seg | seg | seg | seg | seg | seg | seg |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
-------------------------------------------------
x ok ok <ok>| ok ok ok <x>
\\
snd_nxt
1) make veth not return an error when it lost a packet.
While this is what I think we did in the past, the issue keeps
reappearing and it's annoying to debug. The game of whack
a mole is not great.
2) fix the damn return codes
We only talk about NETDEV_TX_OK and NETDEV_TX_BUSY in the
documentation, so maybe we should make the return code from
ndo_start_xmit() a boolean. I like that the most, but perhaps
some ancient, not-really-networking protocol would suffer.
3) make TCP ignore the errors
It is not entirely clear to me what benefit TCP gets from
interpreting the result of ip_queue_xmit()? Specifically once
the connection is established and we're pushing data - packet
loss is just packet loss?
4) this fix
Ignore the rc in the Qdisc-less+GSO case, since it's unreliable.
We already always return OK in the TCQ_F_CAN_BYPASS case.
In the Qdisc-less case let's be a bit more conservative and only
mask the GSO errors. This path is taken by non-IP-"networks"
like CAN, MCTP etc, so we could regress some ancient thing.
This is the simplest, but also maybe the hackiest fix?CVSS
- Versión: 3.1
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Puntuación base: 7.5
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.76%
- Percentil entre todas las CVEs puntuadas: 54
- Fecha de la puntuación: 6/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
🎯 Técnicas ATT&CK
Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.
- Explotación
T1190Exploit Public-Facing Applicationinitial access60 % - Impacto principal
T1499Endpoint Denial of Serviceimpact55 %
Inferido por reglas deterministas a partir del vector CVSS y la CWE. Solo orientativo.
🛡️ Mitigaciones ATT&CK que cubren estas técnicas
Tecnologías afectadas (1)
CWE
- NVD-CWE-noinfo
Referencias
- https://git.kernel.org/stable/c/0c9de092ef8c50a7ee9612811566f0aa81d8d7b6
- https://git.kernel.org/stable/c/4cb163e9efcac4cd35c3043e097f25081a5c015c
- https://git.kernel.org/stable/c/56bd32c0edca34041a5c215887fcf562fae2e2db
- https://git.kernel.org/stable/c/7aa767d0d3d04e50ae94e770db7db8197f666970
- https://git.kernel.org/stable/c/9ac6aebef4b4bfc5ed408b0b65645981574bc780
- https://git.kernel.org/stable/c/ae3f627b45fbc3c776a4e484696f3cad7cbb4eca
- https://git.kernel.org/stable/c/c86901d22c89a6bf4e2f013e948aaabc60869893
- https://git.kernel.org/stable/c/ea5d7787635e26ec1194ec7eec0e8e5ae3bd10a5
JSON original (NVD)
Mostrar
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"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet: consume xmit errors of GSO frames\n\nudpgro_frglist.sh and udpgro_bench.sh are the flakiest tests\ncurrently in NIPA. They fail in the same exact way, TCP GRO\ntest stalls occasionally and the test gets killed after 10min.\n\nThese tests use veth to simulate GRO. They attach a trivial\n(\"return XDP_PASS;\") XDP program to the veth to force TSO off\nand NAPI on.\n\nDigging into the failure mode we can see that the connection\nis completely stuck after a burst of drops. The sender's snd_nxt\nis at sequence number N [1], but the receiver claims to have\nreceived (rcv_nxt) up to N + 3 * MSS [2]. Last piece of the puzzle\nis that senders rtx queue is not empty (let's say the block in\nthe rtx queue is at sequence number N - 4 * MSS [3]).\n\nIn this state, sender sends a retransmission from the rtx queue\nwith a single segment, and sequence numbers N-4*MSS:N-3*MSS [3].\nReceiver sees it and responds with an ACK all the way up to\nN + 3 * MSS [2]. But sender will reject this ack as TCP_ACK_UNSENT_DATA\nbecause it has no recollection of ever sending data that far out [1].\nAnd we are stuck.\n\nThe root cause is the mess of the xmit return codes. veth returns\nan error when it can't xmit a frame. We end up with a loss event\nlike this:\n\n -------------------------------------------------\n | GSO super frame 1 | GSO super frame 2 |\n |-----------------------------------------------|\n | seg | seg | seg | seg | seg | seg | seg | seg |\n | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |\n -------------------------------------------------\n x ok ok <ok>| ok ok ok <x>\n \\\\\n\t\t\t snd_nxt\n\n\"x\" means packet lost by veth, and \"ok\" means it went thru.\nSince veth has TSO disabled in this test it sees individual segments.\nSegment 1 is on the retransmit queue and will be resent.\n\nSo why did the sender not advance snd_nxt even tho it clearly did\nsend up to seg 8? tcp_write_xmit() interprets the return code\nfrom the core to mean that data has not been sent at all. Since\nTCP deals with GSO super frames, not individual segment the crux\nof the problem is that loss of a single segment can be interpreted\nas loss of all. TCP only sees the last return code for the last\nsegment of the GSO frame (in <> brackets in the diagram above).\n\nOf course for the problem to occur we need a setup or a device\nwithout a Qdisc. Otherwise Qdisc layer disconnects the protocol\nlayer from the device errors completely.\n\nWe have multiple ways to fix this.\n\n 1) make veth not return an error when it lost a packet.\n While this is what I think we did in the past, the issue keeps\n reappearing and it's annoying to debug. The game of whack\n a mole is not great.\n\n 2) fix the damn return codes\n We only talk about NETDEV_TX_OK and NETDEV_TX_BUSY in the\n documentation, so maybe we should make the return code from\n ndo_start_xmit() a boolean. I like that the most, but perhaps\n some ancient, not-really-networking protocol would suffer.\n\n 3) make TCP ignore the errors\n It is not entirely clear to me what benefit TCP gets from\n interpreting the result of ip_queue_xmit()? Specifically once\n the connection is established and we're pushing data - packet\n loss is just packet loss?\n\n 4) this fix\n Ignore the rc in the Qdisc-less+GSO case, since it's unreliable.\n We already always return OK in the TCQ_F_CAN_BYPASS case.\n In the Qdisc-less case let's be a bit more conservative and only\n mask the GSO errors. This path is taken by non-IP-\"networks\"\n like CAN, MCTP etc, so we could regress some ancient thing.\n This is the simplest, but also maybe the hackiest fix?\n\nSimilar fix has been proposed by Eric in the past but never committed\nbecause original reporter was working with an OOT driver and wasn't\nproviding feedback (see Link)."
}
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"lastModified": "2026-06-17T10:49:07.773",
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