x86/mm/pkeys: Allow kernel to modify user pkey rights register

The Protection Key Rights for User memory (PKRU) is a 32-bit
user-accessible register.  It contains two bits for each
protection key: one to write-disable (WD) access to memory
covered by the key and another to access-disable (AD).

Userspace can read/write the register with the RDPKRU and WRPKRU
instructions.  But, the register is saved and restored with the
XSAVE family of instructions, which means we have to treat it
like a floating point register.

The kernel needs to write to the register if it wants to
implement execute-only memory or if it implements a system call
to change PKRU.

To do this, we need to create a 'pkru_state' buffer, read the old
contents in to it, modify it, and then tell the FPU code that
there is modified data in there so it can (possibly) move the
buffer back in to the registers.

This uses the fpu__xfeature_set_state() function that we defined
in the previous patch.

Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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