The current way of setting and getting posix acls through the generic
xattr interface is error prone and type unsafe. The vfs needs to
interpret and fixup posix acls before storing or reporting it to
userspace. Various hacks exist to make this work. The code is hard to
understand and difficult to maintain in it's current form. Instead of
making this work by hacking posix acls through xattr handlers we are
building a dedicated posix acl api around the get and set inode
operations. This removes a lot of hackiness and makes the codepaths
easier to maintain. A lot of background can be found in [1].

So far posix acls were passed as a void blob to the security and
integrity modules. Some of them like evm then proceed to interpret the
void pointer and convert it into the kernel internal struct posix acl
representation to perform their integrity checking magic. This is
obviously pretty problematic as that requires knowledge that only the
vfs is guaranteed to have and has lead to various bugs. Add a proper
security hook for setting posix acls and pass down the posix acls in
their appropriate vfs format instead of hacking it through a void
pointer stored in the uapi format.

I spent considerate time in the security module and integrity
infrastructure and audited all codepaths. EVM is the only part that
really has restrictions based on the actual posix acl values passed
through it (e.g., i_mode). Before this dedicated hook EVM used to translate
from the uapi posix acl format sent to it in the form of a void pointer
into the vfs format. This is not a good thing. Instead of hacking around in
the uapi struct give EVM the posix acls in the appropriate vfs format and
perform sane permissions checks that mirror what it used to to in the
generic xattr hook.

IMA doesn't have any restrictions on posix acls. When posix acls are
changed it just wants to update its appraisal status to trigger an EVM
revalidation.

The removal of posix acls is equivalent to passing NULL to the posix set
acl hooks. This is the same as before through the generic xattr api.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
Acked-by: Paul Moore <paul@paul-moore.com> (LSM)
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>