[RFC PATCH v2 4/6] x86/pti: don't mark the user PGD with _PAGE_NX.

[Willy Tarreau <w@1wt.eu>]

Since we're going to keep running on the same PGD when returning to
userspace for certain performance-critical tasks, we'll need the user
pages to be executable. So this code disables the extra protection
that was added consisting in marking user pages _PAGE_NX so that this
pgd remains usable for userspace.

Note: it isn't necessarily the best approach, but one way or another
      if we want to be able to return to userspace from the kernel,
      we'll have to have this executable anyway. Another approach
      might consist in using another pgd for userland+kernel but
      the current core really looks like an extra careful measure
      to catch early bugs if any.

Note2: Andy's suggestion to instead dynamically disable NX upon
       page fault seems the most appealing.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Kees Cook <keescook@chromium.org>
---
 arch/x86/mm/pti.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 43d4a4a..9e2dca0 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -135,9 +135,11 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
 	 *  - we don't have NX support
 	 *  - we're clearing the PGD (i.e. the new pgd is not present).
 	 */
+#if 0
 	if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
 	    (__supported_pte_mask & _PAGE_NX))
 		pgd.pgd |= _PAGE_NX;
+#endif
 
 	/* return the copy of the PGD we want the kernel to use: */
 	return pgd;
-- 
1.7.12.1