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

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Date:   Fri, 17 Aug 2018 09:13:51 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     sean.j.christopherson@...el.com
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Peter Anvin <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] x86/speculation/l1tf: Exempt zeroed PTEs from XOR conversion

On Thu, Aug 16, 2018 at 1:47 PM Sean Christopherson
<sean.j.christopherson@...el.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. */