linux-kernel - Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

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Date:   Mon, 20 Nov 2017 12:46:55 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        Andrew Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

On Mon, Nov 20, 2017 at 12:22 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 10 Nov 2017, Dave Hansen wrote:
>>       __set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu), prot);
>> +
>> +     /* CPU 0's mapping is done in kaiser_init() */
>> +     if (cpu) {
>> +             int ret;
>> +
>> +             ret = kaiser_add_mapping((unsigned long) get_cpu_gdt_ro(cpu),
>> +                                      PAGE_SIZE, __PAGE_KERNEL_RO);
>> +             /*
>> +              * We do not have a good way to fail CPU bringup.
>> +              * Just WARN about it and hope we boot far enough
>> +              * to get a good log out.
>> +              */
>
> The GDT fixmap can be set up before the CPU is started. There is no reason
> to do that in cpu_init().
>
>> +
>> +     /*
>> +      * We could theoretically do this in setup_fixmap_gdt().
>> +      * But, we would need to rewrite the above page table
>> +      * allocation code to use the bootmem allocator.  The
>> +      * buddy allocator is not available at the time that we
>> +      * call setup_fixmap_gdt() for CPU 0.
>> +      */
>> +     kaiser_add_user_map_early(get_cpu_gdt_ro(0), PAGE_SIZE,
>> +                               __PAGE_KERNEL_RO | _PAGE_GLOBAL);
>
> This one is needs to stay.

When you rebase on to my latest version, this should change to mapping
the entire cpu_entry_area.

--Andy