Dynamically enabled XSTATE features are by default disabled for all
processes. A process has to request permission to use such a feature.

To support this implement a architecture specific prctl() with the options:

   - ARCH_GET_XCOMP_SUPP

     Copies the supported feature bitmap into the user space provided
     u64 storage. The pointer is handed in via arg2

   - ARCH_GET_XCOMP_PERM

     Copies the process wide permitted feature bitmap into the user space
     provided u64 storage. The pointer is handed in via arg2

   - ARCH_REQ_XCOMP_PERM

     Request permission for a feature set. A feature set can be mapped to a
     facility, e.g. AMX, and can require one or more XSTATE components to
     be enabled.

     The feature argument is the number of the highest XSTATE component
     which is required for a facility to work.

     The request argument is not a user supplied bitmap because that makes
     filtering harder (think seccomp) and even impossible because to
     support 32bit tasks the argument would have to be a pointer.

The permission mechanism works this way:

   Task asks for permission for a facility and kernel checks whether that's
   supported. If supported it does:

     1) Check whether permission has already been granted

     2) Compute the size of the required kernel and user space buffer
        (sigframe) size.

     3) Validate that no task has a sigaltstack installed
        which is smaller than the resulting sigframe size

     4) Add the requested feature bit(s) to the permission bitmap of
        current->group_leader->fpu and store the sizes in the group
        leaders fpu struct as well.

If that is successful then the feature is still not enabled for any of the
tasks. The first usage of a related instruction will result in a #NM
trap. The trap handler validates the permission bit of the tasks group
leader and if permitted it installs a larger kernel buffer and transfers
the permission and size info to the new fpstate container which makes all
the FPU functions which require per task information aware of the extended
feature set.

  [ tglx: Adopted to new base code, added missing serialization,
          massaged namings, comments and changelog ]
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211021225527.10184-7-chang.seok.bae@intel.com