linux-kernel - Re: [PATCH v3 3/3] x86/asm: Pin sensitive CR0 bits

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Date:   Tue, 18 Jun 2019 11:38:02 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 3/3] x86/asm: Pin sensitive CR0 bits

On Tue, Jun 18, 2019 at 6:55 AM Kees Cook <keescook@...omium.org> wrote:
> With sensitive CR4 bits pinned now, it's possible that the WP bit for
> CR0 might become a target as well. Following the same reasoning for
> the CR4 pinning, this pins CR0's WP bit (but this can be done with a
> static value).
>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/x86/include/asm/special_insns.h | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index c8c8143ab27b..b2e84d113f2a 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -31,7 +31,20 @@ static inline unsigned long native_read_cr0(void)
>
>  static inline void native_write_cr0(unsigned long val)
>  {

So, assuming a legitimate call to native_write_cr0(), we come in here...

> -       asm volatile("mov %0,%%cr0": : "r" (val), "m" (__force_order));
> +       unsigned long bits_missing = 0;
> +
> +set_register:
> +       asm volatile("mov %0,%%cr0": "+r" (val), "+m" (__force_order));

... here we've updated CR0...

> +       if (static_branch_likely(&cr_pinning)) {

... this branch is taken, since cr_pinning is set to true after boot...

> +               if (unlikely((val & X86_CR0_WP) != X86_CR0_WP)) {

... this branch isn't taken, because a legitimate update preserves the WP bit...

> +                       bits_missing = X86_CR0_WP;
> +                       val |= bits_missing;
> +                       goto set_register;
> +               }
> +               /* Warn after we've set the missing bits. */
> +               WARN_ONCE(bits_missing, "CR0 WP bit went missing!?\n");

... and we reach this WARN_ONCE()? Am I missing something, or does
every legitimate CR0 write after early boot now trigger a warning?

> +       }
>  }