Medium [CVE-2026-74483] don't leak the user namespace when the mount fails
This medium-severity Red Hat Linux advisory covers CVE-2026-74483 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat package: kernel.
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Summary
In 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.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · medium-confidence parse· fetched 56 minutes ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · medium-confidence parse· fetched 56 minutes ago·verify at source
Mitigation
The source record does not include mitigation steps. That is not a statement that no fix exists — read the vendor advisory below for the authoritative guidance.
Official advisory · medium-confidence parse· fetched 56 minutes ago·verify at source
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