UBUNTU-CVE-2026-23450: Use After Free

Published Apr 3, 2026

Updated

In the Linux kernel, the following vulnerability has been resolved: net/smc: fix NULL dereference and UAF in smctcpsynrecvsock() Syzkaller reported a panic in smctcpsynrecvsock() [1]. smctcpsynrecvsock() is called in the TCP receive path (softirq) via icskafops->synrecvsock on the clcsock (TCP listening socket). It reads skuserdata to get the smcsock pointer. However, when the SMC listen socket is being closed concurrently, smccloseactive() sets clcsock->skuserdata to NULL under skcallbacklock, and then the smcsock itself can be freed via sockput() in smcrelease(). This leads to two issues: 1) NULL pointer dereference: skuserdata is NULL when accessed. 2) Use-after-free: skuserdata is read as non-NULL, but the smcsock is freed before its fields (e.g., queuedsmchs, oriafops) are accessed. The race window looks like this (the syzkaller crash [1] triggers via the SYN cookie path: tcpgetcookiesock() -> smctcpsynrecvsock(), but the normal tcpcheckreq() path has the same race): CPU A (softirq) CPU B (process ctx) tcpv4rcv() TCPNEWSYNRECV: sk = req->rsklistener sockhold(sk) / No lock on listener / smccloseactive(): writelockbh(cblock) skuserdata = NULL writeunlockbh(cblock) ... smcclcsockrelease() sockput(smc->sk) x2 -> smcsock freed! tcpcheckreq() smctcpsynrecvsock(): smc = userdata(sk) -> NULL or dangling smc->queuedsmchs -> crash! Note that the clcsock and smcsock are two independent objects with separate refcounts. TCP stack holds a reference on the clcsock, which keeps it alive, but this does NOT prevent the smcsock from being freed. Fix this by using RCU and refcountincnotzero() to safely access smcsock. Since smctcpsynrecvsock() is called in the TCP three-way handshake path, taking readlockbh on skcallbacklock is too heavy and would not survive a SYN flood attack. Using rcureadlock() is much more lightweight. - Set SOCKRCUFREE on the SMC listen socket so that smcsock freeing is deferred until after the RCU grace period. This guarantees the memory is still valid when accessed inside rcureadlock(). - Use rcureadlock() to protect reading skuserdata. - Use refcountincnotzero(&smc->sk.skrefcnt) to pin the smcsock. If the refcount has already reached zero (close path completed), it returns false and we bail out safely. Note: smchscongested() has a similar lockless read of skuserdata without rcureadlock(), but it only checks for NULL and accesses the global smchswq, never dereferencing any smcsock field, so it is not affected. Reproducer was verified with mdelay injection and smcrun, the issue no longer occurs with this patch applied. [1] https://syzkaller.appspot.com/bug?extid=827ae2bfb3a3529333e9

Affected Software

186 affected componentsFixes available

ubuntu/linux-hwe-edge*

ubuntu/linux-aws-5.0*

ubuntu/linux-aws-5.3*

ubuntu/linux-azure*

ubuntu/linux-azure-5.3*

ubuntu/linux-azure-edge*

ubuntu/linux-gcp*

ubuntu/linux-gcp-5.3*

ubuntu/linux-gke-4.15*

ubuntu/linux-gke-5.4*

ubuntu/linux-gkeop-5.4*

ubuntu/linux-hwe*

ubuntu/linux-hwe-edge*

ubuntu/linux-oem*

ubuntu/linux-oracle-5.0*

ubuntu/linux-oracle-5.3*

ubuntu/linux-aws-5.11*

ubuntu/linux-aws-5.13*

ubuntu/linux-aws-5.15<5.15.0-1111.118~20.04.1

5.15.0-1111.118~20.04.1

ubuntu/linux-aws-5.8*

ubuntu/linux-azure-5.11*

ubuntu/linux-azure-5.13*

ubuntu/linux-azure-5.15<5.15.0-1116.125~20.04.1

5.15.0-1116.125~20.04.1

ubuntu/linux-azure-5.8*

ubuntu/linux-azure-fde*

ubuntu/linux-azure-fde-5.15<5.15.0-1116.125~20.04.1

5.15.0-1116.125~20.04.1

ubuntu/linux-gcp-5.11*

ubuntu/linux-gcp-5.13*

ubuntu/linux-gcp-5.15<5.15.0-1111.121~20.04.1

5.15.0-1111.121~20.04.1

ubuntu/linux-gcp-5.8*

ubuntu/linux-gke*

ubuntu/linux-gke-5.15*

ubuntu/linux-gkeop*

ubuntu/linux-gkeop-5.15*

ubuntu/linux-hwe-5.11*

ubuntu/linux-hwe-5.13*

ubuntu/linux-hwe-5.15<5.15.0-185.195~20.04.1

5.15.0-185.195~20.04.1

ubuntu/linux-hwe-5.8*

ubuntu/linux-ibm-5.15<5.15.0-1105.109~20.04.1

5.15.0-1105.109~20.04.1

ubuntu/linux-intel-5.13*

ubuntu/linux-intel-iotg-5.15<5.15.0-1106.112~20.04.1

5.15.0-1106.112~20.04.1

ubuntu/linux-lowlatency-hwe-5.15<5.15.0-183.193~20.04.1

5.15.0-183.193~20.04.1

ubuntu/linux-nvidia-tegra-5.15<5.15.0-1063.63~20.04.1

5.15.0-1063.63~20.04.1

ubuntu/linux-oem-5.10*

ubuntu/linux-oem-5.13*

ubuntu/linux-oem-5.6*

ubuntu/linux-oracle-5.11*

ubuntu/linux-oracle-5.13*

ubuntu/linux-oracle-5.15<5.15.0-1108.114~20.04.1

5.15.0-1108.114~20.04.1

ubuntu/linux-oracle-5.8*

ubuntu/linux-raspi2*

ubuntu/linux-riscv*

ubuntu/linux-riscv-5.11*

ubuntu/linux-riscv-5.15<5.15.0-1105.109~20.04.1

5.15.0-1105.109~20.04.1

ubuntu/linux-riscv-5.8*

ubuntu/linux<5.15.0-185.195

5.15.0-185.195

ubuntu/linux-allwinner-5.19*

ubuntu/linux-aws<5.15.0-1111.118

5.15.0-1111.118

ubuntu/linux-aws-5.19*

ubuntu/linux-aws-6.2*

ubuntu/linux-aws-6.5*

ubuntu/linux-aws-6.8<6.8.0-1060.63~22.04.1

6.8.0-1060.63~22.04.1

ubuntu/linux-azure<5.15.0-1116.125

5.15.0-1116.125

ubuntu/linux-azure-5.19*

ubuntu/linux-azure-6.2*

ubuntu/linux-azure-6.5*

ubuntu/linux-azure-6.8<6.8.0-1063.71~22.04.1

6.8.0-1063.71~22.04.1

ubuntu/linux-azure-fde<5.15.0-1116.125

5.15.0-1116.125

ubuntu/linux-azure-fde-5.19*

ubuntu/linux-azure-fde-6.2*

ubuntu/linux-azure-fde-6.8<6.8.0-1062.69~22.04.1

6.8.0-1062.69~22.04.1

ubuntu/linux-gcp<5.15.0-1111.121

5.15.0-1111.121

ubuntu/linux-gcp-5.19*

ubuntu/linux-gcp-6.2*

ubuntu/linux-gcp-6.5*

ubuntu/linux-gcp-6.8<6.8.0-1063.69~22.04.1

6.8.0-1063.69~22.04.1

ubuntu/linux-gke<5.15.0-1107.113

5.15.0-1107.113

ubuntu/linux-gkeop<5.15.0-1094.102

5.15.0-1094.102

ubuntu/linux-hwe-5.19*

ubuntu/linux-hwe-6.2*

ubuntu/linux-hwe-6.5*

ubuntu/linux-hwe-6.8<6.8.0-136.136~22.04.1

6.8.0-136.136~22.04.1

ubuntu/linux-ibm<5.15.0-1105.109

5.15.0-1105.109

ubuntu/linux-ibm-6.8<6.8.0-1060.61~22.04.1

6.8.0-1060.61~22.04.1

ubuntu/linux-intel-iot-realtime*

ubuntu/linux-intel-iotg<5.15.0-1106.112

5.15.0-1106.112

ubuntu/linux-kvm<5.15.0-1103.108

5.15.0-1103.108

ubuntu/linux-lowlatency<5.15.0-183.193

5.15.0-183.193

ubuntu/linux-lowlatency-hwe-5.19*

ubuntu/linux-lowlatency-hwe-6.2*

ubuntu/linux-lowlatency-hwe-6.5*

ubuntu/linux-lowlatency-hwe-6.8<6.8.0-134.134.1~22.04.1

6.8.0-134.134.1~22.04.1

ubuntu/linux-nvidia<5.15.0-1106.107

5.15.0-1106.107

ubuntu/linux-nvidia-6.2*

ubuntu/linux-nvidia-6.5*

ubuntu/linux-nvidia-6.8<6.8.0-1058.61~22.04.1

6.8.0-1058.61~22.04.1

ubuntu/linux-nvidia-tegra<5.15.0-1063.63

5.15.0-1063.63

ubuntu/linux-nvidia-tegra-igx<5.15.0-1052.52

5.15.0-1052.52

ubuntu/linux-oem-5.17*

ubuntu/linux-oem-6.0*

ubuntu/linux-oem-6.1*

ubuntu/linux-oem-6.5*

ubuntu/linux-oracle<5.15.0-1108.114

5.15.0-1108.114

ubuntu/linux-oracle-6.5*

ubuntu/linux-oracle-6.8<6.8.0-1057.58~22.04.1

6.8.0-1057.58~22.04.1

ubuntu/linux-raspi<5.15.0-1105.108

5.15.0-1105.108

ubuntu/linux-realtime*

ubuntu/linux-riscv*

ubuntu/linux-riscv-5.19*

ubuntu/linux-riscv-6.5*

ubuntu/linux-riscv-6.8<6.8.0-134.134~22.04.1

6.8.0-134.134~22.04.1

ubuntu/linux-starfive-5.19*

ubuntu/linux-starfive-6.2*

ubuntu/linux-starfive-6.5*

ubuntu/linux-xilinx-zynqmp<5.15.0-1074.78

5.15.0-1074.78

ubuntu/linux-bluefield<5.15.0-1095.97

5.15.0-1095.97

ubuntu/linux-intel-iot-realtime<5.15.0-1103.105

5.15.0-1103.105

ubuntu/linux-realtime<5.15.0-1110.119

5.15.0-1110.119

ubuntu/linux-realtime-6.8<6.8.1-1055.56~22.04.1

6.8.1-1055.56~22.04.1

ubuntu/linux-aws-fips<5.15.0-1111.118+fips1

5.15.0-1111.118+fips1

ubuntu/linux-azure-fips<5.15.0-1116.125+fips1

5.15.0-1116.125+fips1

ubuntu/linux-fips<5.15.0-185.195+fips1

5.15.0-185.195+fips1

ubuntu/linux-gcp-fips<5.15.0-1111.121+fips1

5.15.0-1111.121+fips1

ubuntu/linux<6.8.0-134.134

6.8.0-134.134

ubuntu/linux-aws<6.8.0-1060.63

6.8.0-1060.63

ubuntu/linux-aws-6.14*

ubuntu/linux-aws-6.17<6.17.0-1019.19~24.04.1

6.17.0-1019.19~24.04.1

ubuntu/linux-azure<6.8.0-1063.71

6.8.0-1063.71

ubuntu/linux-azure-6.11*

ubuntu/linux-azure-6.14*

ubuntu/linux-azure-6.17<6.17.0-1021.21~24.04.1

6.17.0-1021.21~24.04.1

ubuntu/linux-azure-fde<6.8.0-1062.69

6.8.0-1062.69

ubuntu/linux-azure-fde-6.14*

ubuntu/linux-azure-nvidia*

ubuntu/linux-azure-nvidia-6.14*

ubuntu/linux-gcp<6.8.0-1063.69

6.8.0-1063.69

ubuntu/linux-gcp-6.11*

ubuntu/linux-gcp-6.14*

ubuntu/linux-gcp-6.17<6.17.0-1020.22~24.04.1

6.17.0-1020.22~24.04.1

ubuntu/linux-gke<6.8.0-1058.64

6.8.0-1058.64

ubuntu/linux-gkeop<6.8.0-1045.48

6.8.0-1045.48

ubuntu/linux-hwe-6.11*

ubuntu/linux-hwe-6.14*

ubuntu/linux-hwe-6.17<6.17.0-40.40~24.04.1

6.17.0-40.40~24.04.1

ubuntu/linux-ibm<6.8.0-1060.61

6.8.0-1060.61

ubuntu/linux-lowlatency<6.8.0-134.134.1

6.8.0-134.134.1

ubuntu/linux-lowlatency-hwe-6.11*

ubuntu/linux-nvidia<6.8.0-1058.61

6.8.0-1058.61

ubuntu/linux-nvidia-6.11*

ubuntu/linux-nvidia-6.17<6.17.0-1026.26

6.17.0-1026.26

ubuntu/linux-nvidia-lowlatency<6.8.0-1058.61.1

6.8.0-1058.61.1

ubuntu/linux-nvidia-tegra<6.8.0-1029.30

6.8.0-1029.30

ubuntu/linux-oem-6.11*

ubuntu/linux-oem-6.14*

ubuntu/linux-oem-6.17<6.17.0-1028.28

6.17.0-1028.28

ubuntu/linux-oem-6.8*

ubuntu/linux-oracle<6.8.0-1057.58

6.8.0-1057.58

ubuntu/linux-oracle-6.14*

ubuntu/linux-oracle-6.17<6.17.0-1018.18~24.04.1

6.17.0-1018.18~24.04.1

ubuntu/linux-raspi<6.8.0-1060.64

6.8.0-1060.64

ubuntu/linux-raspi-realtime*

ubuntu/linux-realtime*

ubuntu/linux-riscv*

ubuntu/linux-riscv-6.14*

ubuntu/linux-riscv-6.17*

ubuntu/linux-vmware<6.8.0-1008.8

6.8.0-1008.8

ubuntu/linux-xilinx<6.8.0-1032.33

6.8.0-1032.33

ubuntu/linux-bluefield<6.8.0-1026.30

6.8.0-1026.30

ubuntu/linux-raspi-realtime<6.8.0-2049.50

6.8.0-2049.50

ubuntu/linux-realtime<6.8.1-1055.56

6.8.1-1055.56

ubuntu/linux-realtime-6.14*

ubuntu/linux-realtime-6.17<6.17.0-1017.19~24.04.1

6.17.0-1017.19~24.04.1

ubuntu/linux-aws-fips<6.8.0-1060.63+fips1

6.8.0-1060.63+fips1

ubuntu/linux-azure-fips<6.8.0-1062.69+fips1

6.8.0-1062.69+fips1

ubuntu/linux-fips<6.8.0-134.134+fips1

6.8.0-134.134+fips1

ubuntu/linux-gcp-fips<6.8.0-1063.69+fips1

6.8.0-1063.69+fips1

ubuntu/linux<6.17.0-40.40

6.17.0-40.40

ubuntu/linux-aws<6.17.0-1019.19

6.17.0-1019.19

ubuntu/linux-azure*

ubuntu/linux-azure-fde*

ubuntu/linux-gcp<6.17.0-1020.22

6.17.0-1020.22

ubuntu/linux-oracle<6.17.0-1018.18

6.17.0-1018.18

ubuntu/linux-raspi<6.17.0-1021.21

6.17.0-1021.21

ubuntu/linux-realtime<6.17.0-1017.19

6.17.0-1017.19

ubuntu/linux-riscv<6.17.0-40.40.1

6.17.0-40.40.1

ubuntu/linux-vmware*

Remediation

Recommended actions to resolve this vulnerability, in priority order.

Upgrade
Upgrade ubuntu/linux-aws-5.15 to a version that resolves this vulnerability.
Fixed in 5.15.0-1111.118~20.04.1
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Fixed in 5.15.0-1116.125~20.04.1
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Fixed in 5.15.0-1116.125~20.04.1
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Fixed in 5.15.0-1111.121~20.04.1
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Fixed in 5.15.0-185.195~20.04.1
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Fixed in 5.15.0-1105.109~20.04.1
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Fixed in 5.15.0-1106.112~20.04.1
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Fixed in 5.15.0-183.193~20.04.1
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Fixed in 5.15.0-1063.63~20.04.1
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Fixed in 5.15.0-1108.114~20.04.1
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Fixed in 5.15.0-1105.109~20.04.1
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Fixed in 5.15.0-185.195
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Fixed in 5.15.0-1111.118
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Fixed in 6.8.0-1060.63~22.04.1
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Fixed in 5.15.0-1116.125
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Fixed in 6.8.0-1063.71~22.04.1
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Fixed in 5.15.0-1116.125
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Fixed in 6.8.0-1062.69~22.04.1
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Fixed in 5.15.0-1111.121
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Fixed in 6.8.0-1063.69~22.04.1
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Fixed in 5.15.0-1107.113
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Fixed in 5.15.0-1094.102
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Fixed in 6.8.0-136.136~22.04.1
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Fixed in 5.15.0-1105.109
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Fixed in 6.8.0-1060.61~22.04.1
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Fixed in 5.15.0-1106.112
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Fixed in 5.15.0-1103.108
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Fixed in 5.15.0-183.193
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Fixed in 6.8.0-134.134.1~22.04.1
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Fixed in 5.15.0-1106.107
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Fixed in 6.8.0-1058.61~22.04.1
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Fixed in 5.15.0-1063.63
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Fixed in 5.15.0-1052.52
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Fixed in 5.15.0-1108.114
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Fixed in 6.8.0-1057.58~22.04.1
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Fixed in 5.15.0-1105.108
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Fixed in 6.8.0-134.134~22.04.1
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Fixed in 5.15.0-1074.78
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Fixed in 5.15.0-1095.97
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Fixed in 5.15.0-1103.105
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Fixed in 5.15.0-1110.119
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Upgrade ubuntu/linux-realtime-6.8 to a version that resolves this vulnerability.
Fixed in 6.8.1-1055.56~22.04.1
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Fixed in 5.15.0-1111.118+fips1
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Fixed in 5.15.0-1116.125+fips1
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Fixed in 5.15.0-185.195+fips1
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Fixed in 5.15.0-1111.121+fips1
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Fixed in 6.8.0-134.134
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Fixed in 6.8.0-1060.63
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Upgrade ubuntu/linux-aws-6.17 to a version that resolves this vulnerability.
Fixed in 6.17.0-1019.19~24.04.1
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Fixed in 6.8.0-1063.71
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Fixed in 6.17.0-1021.21~24.04.1
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Fixed in 6.8.0-1062.69
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Fixed in 6.8.0-1063.69
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Fixed in 6.17.0-1020.22~24.04.1
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Fixed in 6.8.0-1058.64
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Fixed in 6.8.0-1045.48
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Fixed in 6.17.0-40.40~24.04.1
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Fixed in 6.8.0-1060.61
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Fixed in 6.8.0-134.134.1
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Upgrade ubuntu/linux-nvidia to a version that resolves this vulnerability.
Fixed in 6.8.0-1058.61
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Fixed in 6.17.0-1026.26
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Fixed in 6.8.0-1058.61.1
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Fixed in 6.8.0-1029.30
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Fixed in 6.17.0-1028.28
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Fixed in 6.8.0-1057.58
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Fixed in 6.17.0-1018.18~24.04.1
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Fixed in 6.8.0-1060.64
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Fixed in 6.8.0-1008.8
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Fixed in 6.8.0-1032.33
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Upgrade ubuntu/linux-bluefield to a version that resolves this vulnerability.
Fixed in 6.8.0-1026.30
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Upgrade ubuntu/linux-raspi-realtime to a version that resolves this vulnerability.
Fixed in 6.8.0-2049.50
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Fixed in 6.8.1-1055.56
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Fixed in 6.17.0-1017.19~24.04.1
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Fixed in 6.8.0-1060.63+fips1
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Fixed in 6.8.0-1062.69+fips1
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Fixed in 6.8.0-134.134+fips1
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Fixed in 6.8.0-1063.69+fips1
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Fixed in 6.17.0-40.40
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Fixed in 6.17.0-1019.19
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Fixed in 6.17.0-1020.22
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Fixed in 6.17.0-1018.18
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Fixed in 6.17.0-1021.21
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Fixed in 6.17.0-1017.19
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Fixed in 6.17.0-40.40.1
Configuration
Set SOCK_RCU_FREE on the SMC listen socket so smc_sock freeing is deferred until after the RCU grace period.
Linux kernel SMC listen socket SOCK_RCU_FREE = enabled
Compensating control
Use rcu_read_lock() when reading sk_user_data in smc_tcp_syn_recv_sock() to safely access the smc_sock pointer.
Compensating control
Use refcount_inc_not_zero(&smc->sk.sk_refcnt) to pin the smc_sock before accessing its fields; if it returns false because the close path has completed, bail out safely.

Event History

Apr 3, 2026

CVE Published

via OSV·04:16 PM

Data Sourced

via OSV·04:16 PM

DescriptionSeverityAffected Software

Frequently Asked Questions

Which connection-handling paths are involved in the race?

The issue can be reached through both the SYN cookie path, via tcp_get_cookie_sock() to smc_tcp_syn_recv_sock(), and the normal tcp_check_req() path.

What condition is required to trigger the flaw?

TCP receive processing must access the SMC socket associated with a listening TCP socket while that SMC listen socket is concurrently being closed. During closure, sk_user_data can be cleared and the SMC socket can be freed before receive-path code accesses its fields.

What is the observable impact if the race occurs?

The race can cause either a NULL pointer dereference or a use-after-free in smc_tcp_syn_recv_sock(). Syzkaller reported a kernel panic from this condition.

UBUNTU-CVE-2026-23450: Use After Free

Affected Software

Remediation

Event History

Frequently Asked Questions

Which connection-handling paths are involved in the race?

What condition is required to trigger the flaw?

What is the observable impact if the race occurs?

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