ghsa-gpj2-qxrh-r55r
Vulnerability from github
Published
2024-08-21 03:31
Modified
2024-09-03 15:30
Severity ?
7.0 (High) - CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
Details

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

exec: Fix ToCToU between perm check and set-uid/gid usage

When opening a file for exec via do_filp_open(), permission checking is done against the file's metadata at that moment, and on success, a file pointer is passed back. Much later in the execve() code path, the file metadata (specifically mode, uid, and gid) is used to determine if/how to set the uid and gid. However, those values may have changed since the permissions check, meaning the execution may gain unintended privileges.

For example, if a file could change permissions from executable and not set-id:

---------x 1 root root 16048 Aug 7 13:16 target

to set-id and non-executable:

---S------ 1 root root 16048 Aug 7 13:16 target

it is possible to gain root privileges when execution should have been disallowed.

While this race condition is rare in real-world scenarios, it has been observed (and proven exploitable) when package managers are updating the setuid bits of installed programs. Such files start with being world-executable but then are adjusted to be group-exec with a set-uid bit. For example, "chmod o-x,u+s target" makes "target" executable only by uid "root" and gid "cdrom", while also becoming setuid-root:

-rwxr-xr-x 1 root cdrom 16048 Aug 7 13:16 target

becomes:

-rwsr-xr-- 1 root cdrom 16048 Aug 7 13:16 target

But racing the chmod means users without group "cdrom" membership can get the permission to execute "target" just before the chmod, and when the chmod finishes, the exec reaches brpm_fill_uid(), and performs the setuid to root, violating the expressed authorization of "only cdrom group members can setuid to root".

Re-check that we still have execute permissions in case the metadata has changed. It would be better to keep a copy from the perm-check time, but until we can do that refactoring, the least-bad option is to do a full inode_permission() call (under inode lock). It is understood that this is safe against dead-locks, but hardly optimal.

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{
  "affected": [],
  "aliases": [
    "CVE-2024-43882"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-367"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2024-08-21T01:15:12Z",
    "severity": "HIGH"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nexec: Fix ToCToU between perm check and set-uid/gid usage\n\nWhen opening a file for exec via do_filp_open(), permission checking is\ndone against the file\u0027s metadata at that moment, and on success, a file\npointer is passed back. Much later in the execve() code path, the file\nmetadata (specifically mode, uid, and gid) is used to determine if/how\nto set the uid and gid. However, those values may have changed since the\npermissions check, meaning the execution may gain unintended privileges.\n\nFor example, if a file could change permissions from executable and not\nset-id:\n\n---------x 1 root root 16048 Aug  7 13:16 target\n\nto set-id and non-executable:\n\n---S------ 1 root root 16048 Aug  7 13:16 target\n\nit is possible to gain root privileges when execution should have been\ndisallowed.\n\nWhile this race condition is rare in real-world scenarios, it has been\nobserved (and proven exploitable) when package managers are updating\nthe setuid bits of installed programs. Such files start with being\nworld-executable but then are adjusted to be group-exec with a set-uid\nbit. For example, \"chmod o-x,u+s target\" makes \"target\" executable only\nby uid \"root\" and gid \"cdrom\", while also becoming setuid-root:\n\n-rwxr-xr-x 1 root cdrom 16048 Aug  7 13:16 target\n\nbecomes:\n\n-rwsr-xr-- 1 root cdrom 16048 Aug  7 13:16 target\n\nBut racing the chmod means users without group \"cdrom\" membership can\nget the permission to execute \"target\" just before the chmod, and when\nthe chmod finishes, the exec reaches brpm_fill_uid(), and performs the\nsetuid to root, violating the expressed authorization of \"only cdrom\ngroup members can setuid to root\".\n\nRe-check that we still have execute permissions in case the metadata\nhas changed. It would be better to keep a copy from the perm-check time,\nbut until we can do that refactoring, the least-bad option is to do a\nfull inode_permission() call (under inode lock). It is understood that\nthis is safe against dead-locks, but hardly optimal.",
  "id": "GHSA-gpj2-qxrh-r55r",
  "modified": "2024-09-03T15:30:39Z",
  "published": "2024-08-21T03:31:53Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2024-43882"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/15469d46ba34559bfe7e3de6659115778c624759"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/368f6985d46657b8b466a421dddcacd4051f7ada"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/90dfbba89ad4f0d9c9744ecbb1adac4aa2ff4f3e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/9b424c5d4130d56312e2a3be17efb0928fec4d64"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/d2a2a4714d80d09b0f8eb6438ab4224690b7121e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/d5c3c7e26275a2d83b894d30f7582a42853a958f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/f50733b45d865f91db90919f8311e2127ce5a0cb"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/f6cfc6bcfd5e1cf76115b6450516ea4c99897ae1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ]
}


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