GHSA-W5MR-GVG4-C3PH
Vulnerability from github – Published: 2026-08-15 15:30 – Updated: 2026-08-23 15:33In the Linux kernel, the following vulnerability has been resolved:
binfmt_misc: don't leak the user namespace when the mount fails
bm_get_tree() takes a reference to the user namespace and hands it to get_tree_keyed() as the sget key. sget_fc() moves that reference into sb->s_fs_info and clears fc->s_fs_info, so from that point on the superblock owns it and bm_free() doesn't see it anymore.
The superblock drops it in ->put_super(). But generic_shutdown_super() only calls ->put_super() from inside the if (sb->s_root) branch, so nothing releases it when bm_fill_super() fails:
-
The kzalloc_obj() failure leaves s_root NULL and the whole branch is skipped.
-
A simple_fill_super() failure in the file loop leaves s_root set, but s_op still points at simple_super_operations, which has no ->put_super(). bm_fill_super() installs s_ops only once simple_fill_super() returned success, and installing it earlier wouldn't help either because simple_fill_super() overwrites s_op.
Either way vfs_get_super() calls deactivate_locked_super() and the reference is gone for good. binfmt_misc mounts are available in a user namespace and both the inode and the dentry cache are SLAB_ACCOUNT, so an unprivileged caller under a tight memory cgroup can fail simple_fill_super() on demand and leak one user namespace per attempt.
Drop the reference in ->kill_sb() instead, which runs unconditionally, the same way nfsd and rpc_pipefs release their keyed s_fs_info.
That also stops ->put_super() from clearing s_fs_info while the superblock is still on @fs_supers. generic_shutdown_super() leaves it there on purpose so that sget_fc() keeps finding it until kill_sb() has run, but a NULL s_fs_info makes test_keyed_super() miss it, so a concurrent mount for the same user namespace skips the grab_super() wait and creates a second superblock for a namespace that is still being torn down.
{
"affected": [],
"aliases": [
"CVE-2026-74483"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-08-15T13:17:53Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nbinfmt_misc: don\u0027t leak the user namespace when the mount fails\n\nbm_get_tree() takes a reference to the user namespace and hands it to\nget_tree_keyed() as the sget key. sget_fc() moves that reference into\nsb-\u003es_fs_info and clears fc-\u003es_fs_info, so from that point on the\nsuperblock owns it and bm_free() doesn\u0027t see it anymore.\n\nThe superblock drops it in -\u003eput_super(). But generic_shutdown_super()\nonly calls -\u003eput_super() from inside the if (sb-\u003es_root) branch, so\nnothing releases it when bm_fill_super() fails:\n\n- The kzalloc_obj() failure leaves s_root NULL and the whole branch is\n skipped.\n\n- A simple_fill_super() failure in the file loop leaves s_root set, but\n s_op still points at simple_super_operations, which has no\n -\u003eput_super(). bm_fill_super() installs s_ops only once\n simple_fill_super() returned success, and installing it earlier\n wouldn\u0027t help either because simple_fill_super() overwrites s_op.\n\nEither way vfs_get_super() calls deactivate_locked_super() and the\nreference is gone for good. binfmt_misc mounts are available in a user\nnamespace and both the inode and the dentry cache are SLAB_ACCOUNT, so\nan unprivileged caller under a tight memory cgroup can fail\nsimple_fill_super() on demand and leak one user namespace per attempt.\n\nDrop the reference in -\u003ekill_sb() instead, which runs unconditionally,\nthe same way nfsd and rpc_pipefs release their keyed s_fs_info.\n\nThat also stops -\u003eput_super() from clearing s_fs_info while the\nsuperblock is still on @fs_supers. generic_shutdown_super() leaves it\nthere on purpose so that sget_fc() keeps finding it until kill_sb() has\nrun, but a NULL s_fs_info makes test_keyed_super() miss it, so a\nconcurrent mount for the same user namespace skips the grab_super()\nwait and creates a second superblock for a namespace that is still\nbeing torn down.",
"id": "GHSA-w5mr-gvg4-c3ph",
"modified": "2026-08-23T15:33:02Z",
"published": "2026-08-15T15:30:34Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-74483"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/867aed6a4848761190d5ebdedf9642648f97bceb"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/87a4eb9bbb3497f749bbac612af22ddaa62d7b0d"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/b8206f516fe7cbe785cf44bf09c17c438d7c3cad"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ffec017158d3aad8a6ffca1dfad63d63bde9caad"
}
],
"schema_version": "1.4.0",
"severity": []
}
Sightings
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