ipc: drop the mqueue mount with kern_unmount() on setup failure

Overview

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

ipc: drop the mqueue mount with kern_unmount() on setup failure

mq_init_ns() creates the mqueue mount with fc_mount_longterm(), so its ->mnt_ns is MNT_NS_INTERNAL. When setting up the mqueue or ipc sysctls or msg_init_ns() fails afterwards, create_ipc_ns() drops it with a plain mntput(). With ->mnt_ns set that takes the fast path of mntput_no_expire(). The count drops to zero and nothing ever kills the mount, so the mount and the mqueue superblock are leaked.

Commit dc55e35f9e81 ("ipc: Store mqueue sysctls in the ipc namespace") added the first failure path after mq_init_ns() and never dropped the mount. Commit 7229d74e5e8c ("ipc: create_ipc_ns: drop mqueue mount on sysctl setup failure") tried to fix that with a mntput() but that doesn't help for a long-term mount.

Use kern_unmount(). free_ipc() open-codes the same thing for a namespace that made it.

Affected

• Linux >= dc55e35f9e810f23dd69cfdc91a3d636023f57a2 < c0620b648321fff654173d238d4a85298ea16d05
• Linux >= dc55e35f9e810f23dd69cfdc91a3d636023f57a2 < f0e48172a59e517e613b73f73cc35f24fd123aeb
• Linux 5.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.