x86: Don't let pgprot_modify() change the page encryption bit

[ Upstream commit 6db73f17c5f155dbcfd5e48e621c706270b84df0 ]

When SEV or SME is enabled and active, vm_get_page_prot() typically
returns with the encryption bit set. This means that users of
pgprot_modify(, vm_get_page_prot()) (mprotect_fixup(), do_mmap()) end up
with a value of vma->vm_pg_prot that is not consistent with the intended
protection of the PTEs.

This is also important for fault handlers that rely on the VMA
vm_page_prot to set the page protection. Fix this by not allowing
pgprot_modify() to change the encryption bit, similar to how it's done
for PAT bits.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lkml.kernel.org/r/20200304114527.3636-2-thomas_os@shipmail.org


Signed-off-by: Sasha Levin <sashal@kernel.org>