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CVE-2025-21922

Estado: ModificadaMedia (5.5)—

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

ppp: Fix KMSAN uninit-value warning with bpf

Syzbot caught an "KMSAN: uninit-value" warning [1], which is caused by the ppp driver not initializing a 2-byte header when using socket filter.

The following code can generate a PPP filter BPF program: ''' struct bpf_program fp; pcap_t *handle; handle = pcap_open_dead(DLT_PPP_PPPD, 65535); pcap_compile(handle, &fp, "ip and outbound", 0, 0); bpf_dump(&fp, 1); ''' Its output is: ''' (000) ldh [2] (001) jeq #0x21 jt 2 jf 5 (002) ldb [0] (003) jeq #0x1 jt 4 jf 5 (004) ret #65535 (005) ret #0 ''' Wen can find similar code at the following link: https://github.com/ppp-project/ppp/blob/master/pppd/options.c#L1680 The maintainer of this code repository is also the original maintainer of the ppp driver.

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As you can see the BPF program skips 2 bytes of data and then reads the 'Protocol' field to determine if it's an IP packet. Then it read the first byte of the first 2 bytes to determine the direction.

The issue is that only the first byte indicating direction is initialized in current ppp driver code while the second byte is not initialized.

For normal BPF programs generated by libpcap, uninitialized data won't be used, so it's not a problem. However, for carefully crafted BPF programs, such as those generated by syzkaller [2], which start reading from offset 0, the uninitialized data will be used and caught by KMSAN.

[1] https://syzkaller.appspot.com/bug?extid=853242d9c9917165d791 [2] https://syzkaller.appspot.com/text?tag=ReproC&x=11994913980000

CVSS

Probabilidad de explotación (EPSS)

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).

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

Referencias

JSON original (NVD)

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      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nppp: Fix KMSAN uninit-value warning with bpf\n\nSyzbot caught an \"KMSAN: uninit-value\" warning [1], which is caused by the\nppp driver not initializing a 2-byte header when using socket filter.\n\nThe following code can generate a PPP filter BPF program:\n'''\nstruct bpf_program fp;\npcap_t *handle;\nhandle = pcap_open_dead(DLT_PPP_PPPD, 65535);\npcap_compile(handle, &fp, \"ip and outbound\", 0, 0);\nbpf_dump(&fp, 1);\n'''\nIts output is:\n'''\n(000) ldh [2]\n(001) jeq #0x21 jt 2 jf 5\n(002) ldb [0]\n(003) jeq #0x1 jt 4 jf 5\n(004) ret #65535\n(005) ret #0\n'''\nWen can find similar code at the following link:\nhttps://github.com/ppp-project/ppp/blob/master/pppd/options.c#L1680\nThe maintainer of this code repository is also the original maintainer\nof the ppp driver.\n\nAs you can see the BPF program skips 2 bytes of data and then reads the\n'Protocol' field to determine if it's an IP packet. Then it read the first\nbyte of the first 2 bytes to determine the direction.\n\nThe issue is that only the first byte indicating direction is initialized\nin current ppp driver code while the second byte is not initialized.\n\nFor normal BPF programs generated by libpcap, uninitialized data won't be\nused, so it's not a problem. However, for carefully crafted BPF programs,\nsuch as those generated by syzkaller [2], which start reading from offset\n0, the uninitialized data will be used and caught by KMSAN.\n\n[1] https://syzkaller.appspot.com/bug?extid=853242d9c9917165d791\n[2] https://syzkaller.appspot.com/text?tag=ReproC&x=11994913980000"
    },
    {
      "lang": "es",
      "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: ppp: Corrección de la advertencia de valor no inicializado de KMSAN con bpf. Syzbot detectó una advertencia \"KMSAN: valor no inicializado\" [1], causada por el controlador ppp que no inicializa una cabecera de 2 bytes al usar un filtro de socket. El siguiente código puede generar un programa BPF con filtro PPP: ''' struct bpf_program fp; pcap_t *handle; handle = pcap_open_dead(DLT_PPP_PPPD, 65535); pcap_compile(handle, &fp, \"ip and outbound\", 0, 0); bpf_dump(&fp, 1); ''' Su salida es: ''' (000) ldh [2] (001) jeq #0x21 jt 2 jf 5 (002) ldb [0] (003) jeq #0x1 jt 4 jf 5 (004) ret #65535 (005) ret #0 ''' Wen puede encontrar código similar en el siguiente enlace: https://github.com/ppp-project/ppp/blob/master/pppd/options.c#L1680 El fabricante de este repositorio de código también es el fabricante original del controlador ppp. Como puede ver, el programa BPF omite 2 bytes de datos y luego lee el campo 'Protocolo' para determinar si es un paquete IP. Luego lee el primer byte de los primeros 2 bytes para determinar la dirección. El problema es que solo se inicializa el primer byte que indica la dirección en el código del controlador ppp actual, mientras que el segundo byte no se inicializa. En los programas BPF normales generados por libpcap, no se utilizan datos no inicializados, por lo que no supone un problema. Sin embargo, en programas BPF cuidadosamente manipulados, como los generados por syzkaller [2], que empiezan a leer desde el desplazamiento 0, KMSAN utilizará y capturará los datos no inicializados. [1] https://syzkaller.appspot.com/bug?extid=853242d9c9917165d791 [2] https://syzkaller.appspot.com/text?tag=ReproC&x=11994913980000"
    }
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  "lastModified": "2026-06-17T08:44:37.150",
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