{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2025-21673",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2025-21673"
  ],
  "published": "2025-01-31T11:31:15Z",
  "summary": "smb: client: fix double free of TCP_Server_Info::hostname",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: fix double free of TCP_Server_Info::hostname\n\nWhen shutting down the server in cifs_put_tcp_session(), cifsd thread\nmight be reconnecting to multiple DFS targets before it realizes it\nshould exit the loop, so @server->hostname can't be freed as long as\ncifsd thread isn't done.  Otherwise the following can happen:\n\n  RIP: 0010:__slab_free+0x223/0x3c0\n  Code: 5e 41 5f c3 cc cc cc cc 4c 89 de 4c 89 cf 44 89 44 24 08 4c 89\n  1c 24 e8 fb cf 8e 00 44 8b 44 24 08 4c 8b 1c 24 e9 5f fe ff ff <0f>\n  0b 41 f7 45 08 00 0d 21 00 0f 85 2d ff ff ff e9 1f ff ff ff 80\n  RSP: 0018:ffffb26180dbfd08 EFLAGS: 00010246\n  RAX: ffff8ea34728e510 RBX: ffff8ea34728e500 RCX: 0000000000800068\n  RDX: 0000000000800068 RSI: 0000000000000000 RDI: ffff8ea340042400\n  RBP: ffffe112041ca380 R08: 0000000000000001 R09: 0000000000000000\n  R10: 6170732e31303000 R11: 70726f632e786563 R12: ffff8ea34728e500\n  R13: ffff8ea340042400 R14: ffff8ea34728e500 R15: 0000000000800068\n  FS: 0000000000000000(0000) GS:ffff8ea66fd80000(0000)\n  000000\n  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033\n  CR2: 00007ffc25376080 CR3: 000000012a2ba001 CR4:\n  PKRU: 55555554\n  Call Trace:\n   <TASK>\n   ? show_trace_log_lvl+0x1c4/0x2df\n   ? show_trace_log_lvl+0x1c4/0x2df\n   ? __reconnect_target_unlocked+0x3e/0x160 [cifs]\n   ? __die_body.cold+0x8/0xd\n   ? die+0x2b/0x50\n   ? do_trap+0xce/0x120\n   ? __slab_free+0x223/0x3c0\n   ? do_error_trap+0x65/0x80\n   ? __slab_free+0x223/0x3c0\n   ? exc_invalid_op+0x4e/0x70\n   ? __slab_free+0x223/0x3c0\n   ? asm_exc_invalid_op+0x16/0x20\n   ? __slab_free+0x223/0x3c0\n   ? extract_hostname+0x5c/0xa0 [cifs]\n   ? extract_hostname+0x5c/0xa0 [cifs]\n   ? __kmalloc+0x4b/0x140\n   __reconnect_target_unlocked+0x3e/0x160 [cifs]\n   reconnect_dfs_server+0x145/0x430 [cifs]\n   cifs_handle_standard+0x1ad/0x1d0 [cifs]\n   cifs_demultiplex_thread+0x592/0x730 [cifs]\n   ? __pfx_cifs_demultiplex_thread+0x10/0x10 [cifs]\n   kthread+0xdd/0x100\n   ? __pfx_kthread+0x10/0x10\n   ret_from_fork+0x29/0x50\n   </TASK>",
  "severity": [
    {
      "type": "CVSS_V3",
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
    }
  ],
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-21673"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/1ea68070338518a1d31ce71e6abfe1b30001b27a"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/a2be5f2ba34d0c6d5ef2624b24e3d852561fcd6a"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/fa2f9906a7b333ba757a7dbae0713d8a5396186e"
    }
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  "database_specific": {
    "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git",
    "upstream_modified": "2026-08-05T12:19:11Z",
    "severity": {
      "source": "kernel CNA",
      "cvss_v3_score": 9.8,
      "cvss_v3_severity": "Critical",
      "scenario": "AV:N - The bug lives in the SMB client's connection/reconnect handling, which processes data from the remote peer \u2014 the remote server controls when the TCP connection drops (forcing reconnect) and controls the DFS referral target list that drives the offending free loop. A compromised/malicious SMB-DFS file server, or an on-path attacker able to reset connections and inject referrals, reaches this over the network.\nAC:L - The attacker controls both sides of the race and the window size: it forces reconnects at will, supplies an arbitrarily long DFS target list, and can point targets at blackholed addresses so each `generic_ip_connect()` blocks for the TCP SYN-retry timeout (~130s) while `msleep(3000)` adds more, and the per-target loop never re-checks `tcpStatus`. The race can be retried unboundedly on every reconnect cycle, and it was hit in real-world operation.\nPR:N - The attacking SMB server holds no privileges on the victim client, and the vulnerable `__reconnect_target_unlocked()` path executes during socket reconnect, before SMB negotiate/session-setup \u2014 i.e. pre-authentication. The teardown side is likewise reachable via timer/workqueue-driven paths (`smb2_reconnect_server`, automount expiry) requiring no local privileges.\nUI:N - For an already-mounted DFS share (fstab/autofs, the normal enterprise deployment), reconnect, referral failover, and automount expiry are entirely server- and timer-driven, so no victim action occurs during the attack. The existing mount is a precondition of the deployment, not an in-attack user step.\nS:U - The double free corrupts the kernel slab allocator and impacts only kernel-managed resources within the same security authority. No VM, IOMMU, or sandbox boundary is crossed.\nC:H - A double free lets the same slab object be handed to two allocation sites, yielding type confusion and an arbitrary-read primitive. Additionally there is a direct leak: `reconn_set_ipaddr_from_hostname()` formats the freed string into a UNC and passes it to the `dns_query()` userspace upcall, exfiltrating reused kernel heap contents to userspace verbatim.\nI:H - Double free is a classic heap-corruption primitive \u2014 freelist manipulation via heap spraying gives a controlled write and enables control-flow hijacking and privilege escalation. The freed object is a small attacker-influenced `kmalloc` string in a heavily-used general-purpose cache, making reclaim straightforward.\nA:H - The reported effect is an immediate slab consistency BUG (`__slab_free` invalid-op) killing the cifsd kthread, and a panic under `panic_on_oops`. Even unexploited, the allocator corruption reliably destabilizes or crashes the whole system."
    }
  }
}