CVE-2026-53341

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

fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh()

may_decode_fh() accesses mount::mnt_ns without holding any locks; that means the mount can concurrently be unmounted, and the mnt_namespace can concurrently be freed after an RCU grace period.

This race can happens as follows, assuming that the mount point was created by open_tree(…, OPEN_TREE_CLONE):

thread 1 thread 2 RCU __do_sys_open_by_handle_at do_handle_open handle_to_path may_decode_fh is_mounted [mount::mnt_ns access] [mount::mnt_ns access] __do_sys_close fput_close_sync __fput dissolve_on_fput umount_tree class_namespace_excl_destructor namespace_unlock free_mnt_ns mnt_ns_tree_remove call_rcu(mnt_ns_release_rcu) mnt_ns_release_rcu mnt_ns_release kfree [mnt_namespace::user_ns access] UAF

Fix it by taking rcu_read_lock() around the mount::mnt_ns access, like in __prepend_path(). Additionally, document the semantics of mount::mnt_ns, and use WRITE_ONCE() for writers that can race with lockless readers.

This bug is unreachable unless one of the following is set:

• CONFIG_PREEMPTION
• CONFIG_RCU_STRICT_GRACE_PERIOD

because it requires an RCU grace period to happen during a syscall without an explicit preemption.

This doesnt seem to have interesting security impact; worst-case, it could leak the result of an integer comparison to userspace (from the level check in cap_capable()), cause an endless loop, or crash the kernel by dereferencing an invalid address.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/15ea8dc42a02259d49dee38a658d40f60fcd75ed
• https://git.kernel.org/stable/c/32138633e51e6db59e474765cf93268c92b42888
• https://git.kernel.org/stable/c/40ab6644b99685755f740b872c00ef40d9aa870e
• https://git.kernel.org/stable/c/a8ed2c29fcfdac78db96c9da4e659c8a513f2a94