GHSA-6G9C-7JCV-4G45
Vulnerability from github – Published: 2025-04-01 18:30 – Updated: 2025-11-03 21:33In 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.
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
{
"affected": [],
"aliases": [
"CVE-2025-21922"
],
"database_specific": {
"cwe_ids": [
"CWE-908"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2025-04-01T16:15:22Z",
"severity": "MODERATE"
},
"details": "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\u0027\u0027\u0027\nstruct bpf_program fp;\npcap_t *handle;\nhandle = pcap_open_dead(DLT_PPP_PPPD, 65535);\npcap_compile(handle, \u0026fp, \"ip and outbound\", 0, 0);\nbpf_dump(\u0026fp, 1);\n\u0027\u0027\u0027\nIts output is:\n\u0027\u0027\u0027\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\u0027\u0027\u0027\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\u0027Protocol\u0027 field to determine if it\u0027s 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\u0027t be\nused, so it\u0027s 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\u0026x=11994913980000",
"id": "GHSA-6g9c-7jcv-4g45",
"modified": "2025-11-03T21:33:23Z",
"published": "2025-04-01T18:30:51Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-21922"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/1eacd47636a9de5bee25d9d5962dc538a82d9f0b"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/2f591cb158807bdcf424f66f1fbfa6e4e50f3757"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3de809a768464528762757e433cd50de35bcb3c1"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/4c2d14c40a68678d885eab4008a0129646805bae"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/4e2191b0fd0c064d37b0db67396216f2d4787e0f"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/8aa8a40c766b3945b40565a70349d5581458ff63"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/c036f5f2680cbdabdbbace86baee3c83721634d6"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d685096c8129c9a92689975193e268945fd21dbf"
},
{
"type": "WEB",
"url": "https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html"
},
{
"type": "WEB",
"url": "https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
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