[PATCH 4.1 62/65] PTI: unbreak EFI old_memmap

[Pavel Tatashin]

From: Jiri Kosina <jkosina@xxxxxxx>

old_memmap's efi_call_phys_prolog() calls set_pgd() with swapper PGD that
has PAGE_USER set, which makes PTI set NX on it, and therefore EFI can't
execute it's code.

Fix that by forcefully clearing _PAGE_NX from the PGD (this can't be done
by the pgprot API).

_PAGE_NX will be automatically reintroduced in efi_call_phys_epilog(), as
_set_pgd() will again notice that this is _PAGE_USER, and set _PAGE_NX on
it.

Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 arch/x86/platform/efi/efi_64.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index 18dfaad71c99..12118bae3caf 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -90,6 +90,12 @@ pgd_t * __init efi_call_phys_prolog(void)
 		save_pgd[pgd] = *pgd_offset_k(pgd * PGDIR_SIZE);
 		vaddress = (unsigned long)__va(pgd * PGDIR_SIZE);
 		set_pgd(pgd_offset_k(pgd * PGDIR_SIZE), *pgd_offset_k(vaddress));
+		/*
+		 * pgprot API doesn't clear it for PGD
+		 *
+		 * Will be brought back automatically in _epilog()
+		 */
+		pgd_offset_k(pgd * PGDIR_SIZE)->pgd &= ~_PAGE_NX;
 	}
 out:
 	__flush_tlb_all();
-- 
2.16.2