GHSA-3FFW-PMR7-C5JP

Vulnerability from github – Published: 2026-10-06 09:31 – Updated: 2026-10-06 09:31
VLAI
Details

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

tcp: do not let tcp_rmem be set below 4096

We can hit a division by zero crash in tcp_rcvbuf_grow() and tcp_rcv_space_adjust():

divide error: 0000 [#1] PREEMPT SMP RIP: 0010:tcp_rcvbuf_grow+0x187/0x450 net/ipv4/tcp_input.c:939 ... grow = div_u64(((u64)rcvwin << 1) * (newval - oldval), oldval);

The division uses oldval = tp->rcvq_space.space as divisor. When tp->rcvq_space.space is zero, this leads to a divide-by-zero exception.

tp->rcvq_space.space is initialized in tcp_init_buffer_space(): tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd, (u32)TCP_INIT_CWND * tp->advmss);

If tcp_rmem[1] is configured to very small values (such as 1), sk->sk_rcvbuf is initialized to 1. Then tcp_full_space(sk), which computes (sk->sk_rcvbuf * scaling_ratio) >> 8, truncates to 0. This sets tp->window_clamp = 0, tp->rcv_ssthresh = 0, and tp->rcvq_space.space = 0. Later, when data arrives and DRS is invoked, tcp_rcvbuf_grow() divides by oldval == 0.

Back in 2015, commit b1cb59cf2efe ("net: sysctl_net_core: check SNDBUF and RCVBUF for min length") ensured that net.core.rmem_default and net.core.rmem_max cannot be set below SOCK_MIN_RCVBUF. Similarly, SO_RCVBUF setsockopt enforces max_t(int, val * 2, SOCK_MIN_RCVBUF).

However, net.ipv4.tcp_rmem still had .extra1 = SYSCTL_ONE, allowing arbitrarily small values.

Because SOCK_MIN_RCVBUF depends on sizeof(struct sk_buff) and cacheline alignment, its value varies across architectures and configuration options. Using a fixed constant of 4096 ensures a predictable, architecture- independent lower bound that is safely above SOCK_MIN_RCVBUF everywhere and matches the documented 4K default.

Fix this by setting tcp_rmem.extra1 to 4096 and updating the documentation.

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{
  "affected": [],
  "aliases": [
    "CVE-2026-98299"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-10-06T09:18:20Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\ntcp: do not let tcp_rmem be set below 4096\n\nWe can hit a division by zero crash in tcp_rcvbuf_grow()\nand tcp_rcv_space_adjust():\n\ndivide error: 0000 [#1] PREEMPT SMP\nRIP: 0010:tcp_rcvbuf_grow+0x187/0x450 net/ipv4/tcp_input.c:939\n...\ngrow = div_u64(((u64)rcvwin \u003c\u003c 1) * (newval - oldval), oldval);\n\nThe division uses oldval = tp-\u003ercvq_space.space as divisor.\nWhen tp-\u003ercvq_space.space is zero, this leads to a divide-by-zero\nexception.\n\ntp-\u003ercvq_space.space is initialized in tcp_init_buffer_space():\n    tp-\u003ercvq_space.space = min3(tp-\u003ercv_ssthresh, tp-\u003ercv_wnd,\n                                (u32)TCP_INIT_CWND * tp-\u003eadvmss);\n\nIf tcp_rmem[1] is configured to very small values (such as 1),\nsk-\u003esk_rcvbuf is initialized to 1. Then tcp_full_space(sk), which\ncomputes (sk-\u003esk_rcvbuf * scaling_ratio) \u003e\u003e 8, truncates to 0.\nThis sets tp-\u003ewindow_clamp = 0, tp-\u003ercv_ssthresh = 0, and\ntp-\u003ercvq_space.space = 0. Later, when data arrives and DRS is invoked,\ntcp_rcvbuf_grow() divides by oldval == 0.\n\nBack in 2015, commit b1cb59cf2efe (\"net: sysctl_net_core: check SNDBUF\nand RCVBUF for min length\") ensured that net.core.rmem_default and\nnet.core.rmem_max cannot be set below SOCK_MIN_RCVBUF. Similarly,\nSO_RCVBUF setsockopt enforces max_t(int, val * 2, SOCK_MIN_RCVBUF).\n\nHowever, net.ipv4.tcp_rmem still had .extra1 = SYSCTL_ONE, allowing\narbitrarily small values.\n\nBecause SOCK_MIN_RCVBUF depends on sizeof(struct sk_buff) and cacheline\nalignment, its value varies across architectures and configuration options.\nUsing a fixed constant of 4096 ensures a predictable, architecture-\nindependent lower bound that is safely above SOCK_MIN_RCVBUF everywhere\nand matches the documented 4K default.\n\nFix this by setting tcp_rmem.extra1 to 4096 and updating the documentation.",
  "id": "GHSA-3ffw-pmr7-c5jp",
  "modified": "2026-10-06T09:31:34Z",
  "published": "2026-10-06T09:31:33Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98299"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/1c4bb940c3bb4325bb88ac1b7eaf5faaa39e7d63"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/60df201dbb19a1bf6478b56d100f7ae1fe90e46a"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/67b83c15bed9eeeb8d1a6dbf88a5f79525970173"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/7898bda1ebc198f5559b301a96dd5398edd4d4d3"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/83a945a529d6e002dd7339c532288a931f463dba"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/879907631b9e70eedb62d76461b29afa106ebe7a"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/8e32532d0fa3191ecf9524b0e6bafa0832e712ba"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/9a4f49bf8d2da4e52e904f60c29cca317bab9b3a"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}



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