FKIE_CVE-2026-74618

Vulnerability from fkie_nvd - Published: 2026-08-22 16:16 - Updated: 2026-10-14 13:10
Severity
Summary
In the Linux kernel, the following vulnerability has been resolved: binfmt_misc: don't warn when the mount is completed from another user namespace fsopen() records the caller's user namespace in fc->user_ns and hands back an ordinary file descriptor. Nothing ties the task that calls fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The fd is inherited across fork() and exec() and it can be passed over a unix socket. Completing a context from another user namespace is allowed on purpose. vfs_cmd_create() authorizes the create with mount_capable(), which for FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns. So an unprivileged task can reach the WARN_ON() in bm_fill_super(): create a user and a mount namespace in a child, call fsopen("binfmt_misc") there, send the fscontext fd to the parent and let the parent issue FSCONFIG_CMD_CREATE. Both namespaces come from a plain unshare(1) and no capability is needed anywhere: WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc] CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn Call Trace: get_tree_keyed+0x7d/0xb0 bm_get_tree+0x34/0x90 [binfmt_misc] vfs_get_tree+0x2a/0x100 vfs_cmd_create+0x60/0xf0 __do_sys_fsconfig+0x4b2/0x500 The child needs the mount namespace because fsopen() itself gates on may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning the caller's mount namespace. fsconfig() doesn't repeat that check. It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be raised in a loop to taint the kernel and flood the log, and it panics a kernel booted with panic_on_warn. Keep refusing the mount and stop warning about it. Nothing in bm_fill_super() depends on the two namespaces matching, it derives everything from sb->s_user_ns.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/047f927f54c6c17593e93aafe82dcb7acdda2a71
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/24e95a24f151ce40d5fc1b3a6cefbcda8ded736c
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/37cf5cf1320a84a17225a1690547b8a0812ca94e
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/79fdf39f1a31f88cb3833b6f8091fbf6acdca2c6
Impacted products
Vendor Product Version
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{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/binfmt_misc.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "37cf5cf1320a84a17225a1690547b8a0812ca94e",
              "status": "affected",
              "version": "21ca59b365c091d583f36ac753eaa8baf947be6f",
              "versionType": "git"
            },
            {
              "lessThan": "24e95a24f151ce40d5fc1b3a6cefbcda8ded736c",
              "status": "affected",
              "version": "21ca59b365c091d583f36ac753eaa8baf947be6f",
              "versionType": "git"
            },
            {
              "lessThan": "047f927f54c6c17593e93aafe82dcb7acdda2a71",
              "status": "affected",
              "version": "21ca59b365c091d583f36ac753eaa8baf947be6f",
              "versionType": "git"
            },
            {
              "lessThan": "79fdf39f1a31f88cb3833b6f8091fbf6acdca2c6",
              "status": "affected",
              "version": "21ca59b365c091d583f36ac753eaa8baf947be6f",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "fs/binfmt_misc.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.7"
            },
            {
              "lessThan": "6.7",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.104",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.45",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.1.*",
              "status": "unaffected",
              "version": "7.1.9",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.2",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nbinfmt_misc: don\u0027t warn when the mount is completed from another user namespace\n\nfsopen() records the caller\u0027s user namespace in fc-\u003euser_ns and hands\nback an ordinary file descriptor. Nothing ties the task that calls\nfsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The\nfd is inherited across fork() and exec() and it can be passed over a\nunix socket.\n\nCompleting a context from another user namespace is allowed on purpose.\nvfs_cmd_create() authorizes the create with mount_capable(), which for\nFS_USERNS_MOUNT checks ns_capable(fc-\u003euser_ns, CAP_SYS_ADMIN), and that\nsucceeds for a task holding CAP_SYS_ADMIN in an ancestor of fc-\u003euser_ns.\nSo an unprivileged task can reach the WARN_ON() in bm_fill_super():\ncreate a user and a mount namespace in a child, call\nfsopen(\"binfmt_misc\") there, send the fscontext fd to the parent and let\nthe parent issue FSCONFIG_CMD_CREATE. Both namespaces come from a plain\nunshare(1) and no capability is needed anywhere:\n\n  WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc]\n  CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn\n  Call Trace:\n   get_tree_keyed+0x7d/0xb0\n   bm_get_tree+0x34/0x90 [binfmt_misc]\n   vfs_get_tree+0x2a/0x100\n   vfs_cmd_create+0x60/0xf0\n   __do_sys_fsconfig+0x4b2/0x500\n\nThe child needs the mount namespace because fsopen() itself gates on\nmay_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning\nthe caller\u0027s mount namespace. fsconfig() doesn\u0027t repeat that check.\n\nIt is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be\nraised in a loop to taint the kernel and flood the log, and it panics a\nkernel booted with panic_on_warn.\n\nKeep refusing the mount and stop warning about it. Nothing in\nbm_fill_super() depends on the two namespaces matching, it derives\neverything from sb-\u003es_user_ns."
    },
    {
      "lang": "es",
      "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nbinfmt_misc: no advertir cuando el montaje se completa desde otro espacio de nombres de usuario\n\nfsopen() registra el espacio de nombres de usuario del llamador en fc -\u003e user_ns y devuelve un descriptor de archivo ordinario. Nada vincula la tarea que llama a fsconfig(FSCONFIG_CMD_CREATE) con la tarea que cre\u00f3 el contexto. El fd se hereda a trav\u00e9s de fork() y exec() y puede pasarse a trav\u00e9s de un socket unix.\n\nCompletar un contexto desde otro espacio de nombres de usuario est\u00e1 permitido a prop\u00f3sito. vfs_cmd_create() autoriza la creaci\u00f3n con mount_capable(), que para FS_USERNS_MOUNT comprueba ns_capable(fc -\u003e user_ns, CAP_SYS_ADMIN), y eso tiene \u00e9xito para una tarea que posee CAP_SYS_ADMIN en un ancestro de fc -\u003e user_ns. As\u00ed, una tarea sin privilegios puede alcanzar el WARN_ON() en bm_fill_super(): crear un espacio de nombres de usuario y de montaje en un hijo, llamar a fsopen(\u0027binfmt_misc\u0027) all\u00ed, enviar el fd de fscontext al padre y dejar que el padre emita FSCONFIG_CMD_CREATE. Ambos espacios de nombres provienen de un simple unshare(1) y no se necesita ninguna capacidad en ning\u00fan lugar:\n\n  WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc]\n  CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn\n  Ruta de Llamada:\n   get_tree_keyed+0x7d/0xb0\n   bm_get_tree+0x34/0x90 [binfmt_misc]\n   vfs_get_tree+0x2a/0x100\n   vfs_cmd_create+0x60/0xf0\n   __do_sys_fsconfig+0x4b2/0x500\n\nEl hijo necesita el espacio de nombres de montaje porque fsopen() mismo se basa en may_mount(), que solicita CAP_SYS_ADMIN en el espacio de nombres de usuario propietario del espacio de nombres de montaje del llamador. fsconfig() no repite esa comprobaci\u00f3n.\n\nEs un WARN_ON() y no un WARN_ON_ONCE(), por lo que la condici\u00f3n puede ser elevada en un bucle para corromper el kernel e inundar el registro, y provoca un p\u00e1nico en un kernel arrancado con panic_on_warn.\n\nSeguir rechazando el montaje y dejar de advertir al respecto. Nada en bm_fill_super() depende de que los dos espacios de nombres coincidan, deriva todo de sb -\u003e s_user_ns."
    }
  ],
  "id": "CVE-2026-74618",
  "lastModified": "2026-10-14T13:10:00.187",
  "metrics": {},
  "published": "2026-08-22T16:16:34.507",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/047f927f54c6c17593e93aafe82dcb7acdda2a71"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/24e95a24f151ce40d5fc1b3a6cefbcda8ded736c"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/37cf5cf1320a84a17225a1690547b8a0812ca94e"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/79fdf39f1a31f88cb3833b6f8091fbf6acdca2c6"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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