GHSA-8FRP-2M2W-WR4J

Vulnerability from github – Published: 2026-05-08 15:31 – Updated: 2026-05-08 15:31
VLAI
Details

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

ceph: fix i_nlink underrun during async unlink

During async unlink, we drop the i_nlink counter before we receive the completion (that will eventually update the i_nlink) because "we assume that the unlink will succeed". That is not a bad idea, but it races against deletions by other clients (or against the completion of our own unlink) and can lead to an underrun which emits a WARNING like this one:

WARNING: CPU: 85 PID: 25093 at fs/inode.c:407 drop_nlink+0x50/0x68 Modules linked in: CPU: 85 UID: 3221252029 PID: 25093 Comm: php-cgi8.1 Not tainted 6.14.11-cm4all1-ampere #655 Hardware name: Supermicro ARS-110M-NR/R12SPD-A, BIOS 1.1b 10/17/2023 pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : drop_nlink+0x50/0x68 lr : ceph_unlink+0x6c4/0x720 sp : ffff80012173bc90 x29: ffff80012173bc90 x28: ffff086d0a45aaf8 x27: ffff0871d0eb5680 x26: ffff087f2a64a718 x25: 0000020000000180 x24: 0000000061c88647 x23: 0000000000000002 x22: ffff07ff9236d800 x21: 0000000000001203 x20: ffff07ff9237b000 x19: ffff088b8296afc0 x18: 00000000f3c93365 x17: 0000000000070000 x16: ffff08faffcbdfe8 x15: ffff08faffcbdfec x14: 0000000000000000 x13: 45445f65645f3037 x12: 34385f6369706f74 x11: 0000a2653104bb20 x10: ffffd85f26d73290 x9 : ffffd85f25664f94 x8 : 00000000000000c0 x7 : 0000000000000000 x6 : 0000000000000002 x5 : 0000000000000081 x4 : 0000000000000481 x3 : 0000000000000000 x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff08727d3f91e8 Call trace: drop_nlink+0x50/0x68 (P) vfs_unlink+0xb0/0x2e8 do_unlinkat+0x204/0x288 __arm64_sys_unlinkat+0x3c/0x80 invoke_syscall.constprop.0+0x54/0xe8 do_el0_svc+0xa4/0xc8 el0_svc+0x18/0x58 el0t_64_sync_handler+0x104/0x130 el0t_64_sync+0x154/0x158

In ceph_unlink(), a call to ceph_mdsc_submit_request() submits the CEPH_MDS_OP_UNLINK to the MDS, but does not wait for completion.

Meanwhile, between this call and the following drop_nlink() call, a worker thread may process a CEPH_CAP_OP_IMPORT, CEPH_CAP_OP_GRANT or just a CEPH_MSG_CLIENT_REPLY (the latter of which could be our own completion). These will lead to a set_nlink() call, updating the i_nlink counter to the value received from the MDS. If that new i_nlink value happens to be zero, it is illegal to decrement it further. But that is exactly what ceph_unlink() will do then.

The WARNING can be reproduced this way:

  1. Force async unlink; only the async code path is affected. Having no real clue about Ceph internals, I was unable to find out why the MDS wouldn't give me the "Fxr" capabilities, so I patched get_caps_for_async_unlink() to always succeed.

(Note that the WARNING dump above was found on an unpatched kernel, without this kludge - this is not a theoretical bug.)

  1. Add a sleep call after ceph_mdsc_submit_request() so the unlink completion gets handled by a worker thread before drop_nlink() is called. This guarantees that the i_nlink is already zero before drop_nlink() runs.

The solution is to skip the counter decrement when it is already zero, but doing so without a lock is still racy (TOCTOU). Since ceph_fill_inode() and handle_cap_grant() both hold the ceph_inode_info.i_ceph_lock spinlock while set_nlink() runs, this seems like the proper lock to protect the i_nlink updates.

I found prior art in NFS and SMB (using inode.i_lock) and AFS (using afs_vnode.cb_lock). All three have the zero check as well.

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{
  "affected": [],
  "aliases": [
    "CVE-2026-43420"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-05-08T15:16:54Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nceph: fix i_nlink underrun during async unlink\n\nDuring async unlink, we drop the `i_nlink` counter before we receive\nthe completion (that will eventually update the `i_nlink`) because \"we\nassume that the unlink will succeed\".  That is not a bad idea, but it\nraces against deletions by other clients (or against the completion of\nour own unlink) and can lead to an underrun which emits a WARNING like\nthis one:\n\n WARNING: CPU: 85 PID: 25093 at fs/inode.c:407 drop_nlink+0x50/0x68\n Modules linked in:\n CPU: 85 UID: 3221252029 PID: 25093 Comm: php-cgi8.1 Not tainted 6.14.11-cm4all1-ampere #655\n Hardware name: Supermicro ARS-110M-NR/R12SPD-A, BIOS 1.1b 10/17/2023\n pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)\n pc : drop_nlink+0x50/0x68\n lr : ceph_unlink+0x6c4/0x720\n sp : ffff80012173bc90\n x29: ffff80012173bc90 x28: ffff086d0a45aaf8 x27: ffff0871d0eb5680\n x26: ffff087f2a64a718 x25: 0000020000000180 x24: 0000000061c88647\n x23: 0000000000000002 x22: ffff07ff9236d800 x21: 0000000000001203\n x20: ffff07ff9237b000 x19: ffff088b8296afc0 x18: 00000000f3c93365\n x17: 0000000000070000 x16: ffff08faffcbdfe8 x15: ffff08faffcbdfec\n x14: 0000000000000000 x13: 45445f65645f3037 x12: 34385f6369706f74\n x11: 0000a2653104bb20 x10: ffffd85f26d73290 x9 : ffffd85f25664f94\n x8 : 00000000000000c0 x7 : 0000000000000000 x6 : 0000000000000002\n x5 : 0000000000000081 x4 : 0000000000000481 x3 : 0000000000000000\n x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff08727d3f91e8\n Call trace:\n  drop_nlink+0x50/0x68 (P)\n  vfs_unlink+0xb0/0x2e8\n  do_unlinkat+0x204/0x288\n  __arm64_sys_unlinkat+0x3c/0x80\n  invoke_syscall.constprop.0+0x54/0xe8\n  do_el0_svc+0xa4/0xc8\n  el0_svc+0x18/0x58\n  el0t_64_sync_handler+0x104/0x130\n  el0t_64_sync+0x154/0x158\n\nIn ceph_unlink(), a call to ceph_mdsc_submit_request() submits the\nCEPH_MDS_OP_UNLINK to the MDS, but does not wait for completion.\n\nMeanwhile, between this call and the following drop_nlink() call, a\nworker thread may process a CEPH_CAP_OP_IMPORT, CEPH_CAP_OP_GRANT or\njust a CEPH_MSG_CLIENT_REPLY (the latter of which could be our own\ncompletion).  These will lead to a set_nlink() call, updating the\n`i_nlink` counter to the value received from the MDS.  If that new\n`i_nlink` value happens to be zero, it is illegal to decrement it\nfurther.  But that is exactly what ceph_unlink() will do then.\n\nThe WARNING can be reproduced this way:\n\n1. Force async unlink; only the async code path is affected.  Having\n   no real clue about Ceph internals, I was unable to find out why the\n   MDS wouldn\u0027t give me the \"Fxr\" capabilities, so I patched\n   get_caps_for_async_unlink() to always succeed.\n\n   (Note that the WARNING dump above was found on an unpatched kernel,\n   without this kludge - this is not a theoretical bug.)\n\n2. Add a sleep call after ceph_mdsc_submit_request() so the unlink\n   completion gets handled by a worker thread before drop_nlink() is\n   called.  This guarantees that the `i_nlink` is already zero before\n   drop_nlink() runs.\n\nThe solution is to skip the counter decrement when it is already zero,\nbut doing so without a lock is still racy (TOCTOU).  Since\nceph_fill_inode() and handle_cap_grant() both hold the\n`ceph_inode_info.i_ceph_lock` spinlock while set_nlink() runs, this\nseems like the proper lock to protect the `i_nlink` updates.\n\nI found prior art in NFS and SMB (using `inode.i_lock`) and AFS (using\n`afs_vnode.cb_lock`).  All three have the zero check as well.",
  "id": "GHSA-8frp-2m2w-wr4j",
  "modified": "2026-05-08T15:31:28Z",
  "published": "2026-05-08T15:31:28Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-43420"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/6d5fd8bb574bef039eb3b738e523870433a2aeb9"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/7db008e85a5d17b64bc5390b828bf457ae91a415"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/8975b85b0d45ca811ace6fac5907652f2310e5ac"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/9b31e88ac5623d15c8bc46f69dfe1d3b43a8f67c"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/aedd29386b23f3e1e6818943e11abfff2953732f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/b3f5513141ecc6b277a8f7b7efe58a0cf9a5e859"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/ce0123cbb4a40a2f1bbb815f292b26e96088639f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/fcc477a6e8856c8a42b3c9e171724d8d6dfadd06"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}


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