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Subject: Re: [patch V163 27/51] x86/mm/pti: Populate user PGD
To: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
References: <20171218114215.239543034@linutronix.de>
 <20171218115255.827361623@linutronix.de>
Cc: x86@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirsky <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bpetkov@suse.de>,
        Greg KH <gregkh@linuxfoundation.org>, keescook@google.com,
        hughd@google.com, Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Denys Vlasenko <dvlasenk@redhat.com>, Rik van Riel <riel@redhat.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        David Laight <David.Laight@aculab.com>,
        Eduardo Valentin <eduval@amazon.com>, aliguori@amazon.com,
        Will Deacon <will.deacon@arm.com>, daniel.gruss@iaik.tugraz.at,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
        "H. Peter Anvin" <hpa@zytor.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <93e88e2c-50fc-49a6-7113-d9b64cf70fbd@intel.com>
Date: Mon, 18 Dec 2017 12:34:22 -0800
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On 12/18/2017 03:42 AM, Thomas Gleixner wrote:
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1120,6 +1120,11 @@ static inline void pmdp_set_wrprotect(st
>  static inline void clone_pgd_range(pgd_t *dst, pgd_t *src, int count)
>  {
>         memcpy(dst, src, count * sizeof(pgd_t));
> +#ifdef CONFIG_PAGE_TABLE_ISOLATION
> +	/* Clone the user space pgd as well */
> +	memcpy(kernel_to_user_pgdp(dst), kernel_to_user_pgdp(src),
> +	       count * sizeof(pgd_t));
> +#endif
>  }

I was just thinking about this as I re-write the documentation about
where the overhead of pti comes from.

This obviously *works* for now.  But, we certainly have the pti-mapped
stuff spread much less through the address space than when this was
thrown in here.  It *seems* like we could probably do this with just 4 PGDs:

>         pti_clone_user_shared();
>         pti_clone_entry_text();
>         pti_setup_espfix64();
>         pti_setup_vsyscall();

The vsyscall is just one page and the espfix is *sized* to be one PGD,
so we know each of those only takes one entry.

We surely don't have 512GB of entry_text, and I don't think KASLR can
ever cause it to span two PGD entries.

I also don't think the user_shared area of the fixmap can get *that*
big.  Does anybody know offhand what the theoretical limits are there?