landlock: Don't lose track of restrictions on cred_transfer

Vulnerability Description

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.)

References

• https://git.kernel.org/stable/c/916c648323fa53b89eedb34a0988ddaf01406117
• https://git.kernel.org/stable/c/0d74fd54db0bd0c0c224bef0da8fc95ea9c9f36c
• https://git.kernel.org/stable/c/16896914bace82d7811c62f3b6d5320132384f49
• https://git.kernel.org/stable/c/b14cc2cf313bd29056fadbc8ecd7f957cf5791ff
• https://git.kernel.org/stable/c/39705a6c29f8a2b93cf5b99528a55366c50014d1
• https://lore.kernel.org/all/20240817.shahka3Ee1iy@digikod.net/
• https://www.openwall.com/lists/oss-security/2024/08/17/2
• https://bugs.chromium.org/p/project-zero/issues/detail?id=2566