unshare: don't drop active namespace references that were never taken

Summary

In the Linux kernel, the following vulnerability has been resolved: unshare: don't drop active namespace references that were never taken Active references on the namespaces of an nsproxy are taken when the nsproxy is installed into a task in switch_task_namespaces() and copy_namespaces() and dropped again by put_nsproxy() through deactivate_nsproxy(). But ksys_unshare() calls put_nsproxy() on an nsproxy that was never installed when set_cred_ucounts() fails. The new namespaces of that nsproxy go from zero to minus one and the namespaces shared with the caller lose a reference that belongs to the nsproxy the caller keeps using: WARNING: kernel/nscommon.c:171 at __ns_ref_active_put+0x1cd/0x230 nsproxy_ns_active_put deactivate_nsproxy ksys_unshare Afterwards the namespaces the caller lives in aren't listed by listns() anymore. set_cred_ucounts() only fails when alloc_ucounts() can't allocate and it only allocates when the real uid of the caller differs from its effective uid. Commit cefd55bd2159 ("nsproxy: fix free_nsproxy() and simplify create_new_namespaces()") separated the two cases on purpose. nsproxy_free() frees an nsproxy that was prepared but never installed and that's what a failed setns() uses in put_nsset(). Export it and use it for a failed unshare() as well.

Timeline

1. Oct 23, 2026, 12:20 PM UTCCVE published
2. Oct 23, 2026, 12:45 PM UTCFirst seen by cve.now

Affected versions

• Linux Linux

  From a98621a0f187a934c115dcfe79a49520ae892111 · fixed in 56fe03a0c05dba4c5f6a23c391f96de1614a098c; From a98621a0f187a934c115dcfe79a49520ae892111 · fixed in 0b6f31a3c0b16066f812e94b52a704174a6f23cd; From 5.15.209 · fixed in 5.16; From 6.1.175 · fixed in 6.2; From 6.6.141 · fixed in 6.7; From 6.12.91 · fixed in 6.13; From 6.18.33 · fixed in 6.19; From 7.0.10 · fixed in 7.1

• Linux Linux

  7.1

• KernelLinux

  From 7.1.0 · fixed in 7.2.11

References 2