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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?
> + }
> }
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