x86/pkeys: Properly copy pkey state at fork()

Memory protection key behavior should be the same in a child as it was
in the parent before a fork.  But, there is a bug that resets the
state in the child at fork instead of preserving it.

The creation of new mm's is a bit convoluted.  At fork(), the code
does:

  1. memcpy() the parent mm to initialize child
  2. mm_init() to initalize some select stuff stuff
  3. dup_mmap() to create true copies that memcpy() did not do right

For pkeys two bits of state need to be preserved across a fork:
'execute_only_pkey' and 'pkey_allocation_map'.

Those are preserved by the memcpy(), but mm_init() invokes
init_new_context() which overwrites 'execute_only_pkey' and
'pkey_allocation_map' with "new" values.

The author of the code erroneously believed that init_new_context is *only*
called at execve()-time.  But, alas, init_new_context() is used at execve()
and fork().

The result is that, after a fork(), the child's pkey state ends up looking
like it does after an execve(), which is totally wrong.  pkeys that are
already allocated can be allocated again, for instance.

To fix this, add code called by dup_mmap() to copy the pkey state from
parent to child explicitly.  Also add a comment above init_new_context() to
make it more clear to the next poor sod what this code is used for.

Fixes: e8c24d3a ("x86/pkeys: Allocation/free syscalls")
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Cc: peterz@infradead.org
Cc: mpe@ellerman.id.au
Cc: will.deacon@arm.com
Cc: luto@kernel.org
Cc: jroedel@suse.de
Cc: stable@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Joerg Roedel <jroedel@suse.de>
Link: https://lkml.kernel.org/r/20190102215655.7A69518C@viggo.jf.intel.com