Re: [PATCH] x86/speculation/l1tf: Exempt zeroed PTEs from XOR conversion

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Aug 16, 2018 at 1:47 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> Fixes: 6b28baca9b1f ("x86/speculation/l1tf: Protect PROT_NONE PTEs against speculation")

This seems wrong.

That commit doesn't invert a cleared page table entry, because that
commit still required _PAGE_PROTNONE being set for a pte to be
inverted.

I'm assuming the real culprit is commit f22cc87f6c1f
("x86/speculation/l1tf: Invert all not present mappings") which made
it look at _just_ the present bit.

And yeah, that was wrong.

So I really think a much better patch would be the appended one-liner.

Note - it's whitespace-damaged by cut-and-paste, but it should be
obvious enough to apply by hand.

Can you test this one instead?

             Linus
---

 arch/x86/include/asm/pgtable-invert.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable-invert.h
b/arch/x86/include/asm/pgtable-invert.h
index 44b1203ece12..821438e91b77 100644
--- a/arch/x86/include/asm/pgtable-invert.h
+++ b/arch/x86/include/asm/pgtable-invert.h
@@ -6,7 +6,7 @@

 static inline bool __pte_needs_invert(u64 val)
 {
-       return !(val & _PAGE_PRESENT);
+       return val && !(val & _PAGE_PRESENT);
 }

 /* Get a mask to xor with the page table entry to get the correct pfn. */