CVE-2024-42318

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

landlock: Don't lose track of restrictions on cred_transfer

When a process' cred struct is replaced, this _almost_ always invokes

the cred_prepare LSM hook; but in one special case (when

KEYCTL_SESSION_TO_PARENT updates the parent's credentials), the

cred_transfer LSM hook is used instead. Landlock only implements the

cred_prepare hook, not cred_transfer, so KEYCTL_SESSION_TO_PARENT causes

all information on Landlock restrictions to be lost.

This basically means that a process with the ability to use the fork()

and keyctl() syscalls can get rid of all Landlock restrictions on

itself.

Fix it by adding a cred_transfer hook that does the same thing as the

existing cred_prepare hook. (Implemented by having hook_cred_prepare()

call hook_cred_transfer() so that the two functions are less likely to

accidentally diverge in the future.)