Re: [Xen-devel] [PATCH v2 3/3] x86: Make the GDT remapping read-only on 64 bit

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Wed, Feb 1, 2017 at 9:14 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Thu, Jan 26, 2017 at 8:59 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>> This patch makes the GDT remapped pages read-only to prevent corruption.
>> This change is done only on 64 bit.
>>
>> The native_load_tr_desc function was adapted to correctly handle a
>> read-only GDT. The LTR instruction always writes to the GDT TSS entry.
>> This generates a page fault if the GDT is read-only. This change checks
>> if the current GDT is a remap and swap GDTs as needed. This function was
>> tested by booting multiple machines and checking hibernation works
>> properly.
>>
>> KVM SVM and VMX were adapted to use the writeable GDT. On VMX, the
>> per-cpu variable was removed for functions to fetch the original GDT.
>> Instead of reloading the previous GDT, VMX will reload the fixmap GDT as
>> expected. For testing, VMs were started and restored on multiple
>> configurations.
>>
>> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
>> ---
>> Based on next-20170125
>> ---
>>  arch/x86/include/asm/desc.h      | 46 
>> +++++++++++++++++++++++++++++++++++-----
>>  arch/x86/include/asm/processor.h |  1 +
>>  arch/x86/kernel/cpu/common.c     | 28 ++++++++++++++++++------
>>  arch/x86/kvm/svm.c               |  4 +---
>>  arch/x86/kvm/vmx.c               | 15 +++++--------
>>  5 files changed, 70 insertions(+), 24 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
>> index 4cc176f57b78..ca7b2224fcb4 100644
>> --- a/arch/x86/include/asm/desc.h
>> +++ b/arch/x86/include/asm/desc.h
>> @@ -52,6 +52,12 @@ static inline struct desc_struct 
>> *get_cpu_direct_gdt(unsigned int cpu)
>>         return per_cpu(gdt_page, cpu).gdt;
>>  }
>>
>> +/* Provide the current original GDT */
>> +static inline struct desc_struct *get_current_direct_gdt(void)
>> +{
>> +       return this_cpu_ptr(&gdt_page)->gdt;
>> +}
>
> I'm assuming that the reason that this isn't part of patch 2 and used
> instead of the version that takes cpu as a parameter is that TLS
> doesn't work until the GDT is set up.  If so, perhaps that's worthy of
> a comment in patch 2.
>
> But give this_cpu_read(gdt_page.gdt) a try, please.
>

I tried but I can't get it working properly because the gdt field is
an array, not a pointer. For example with this_cpu_read(gdt_page.gdt),
I get:

./arch/x86/include/asm/desc.h: In function ‘get_current_gdt_rw’:
./include/linux/percpu-defs.h:308:21: error: incompatible types when
assigning to type ‘struct desc_struct[16]’ from type ‘struct
desc_struct *’
  case 1: pscr_ret__ = stem##1(variable); break;   \
                     ^
I tried different variants without success. What do you think?

>> +/*
>> + * The LTR instruction marks the TSS GDT entry as busy. In 64bit, the GDT is
>> + * a read-only remapping. To prevent a page fault, the GDT is switched to 
>> the
>> + * original writeable version when needed.
>> + */
>> +#ifdef CONFIG_X86_64
>> +static inline void native_load_tr_desc(void)
>> +{
>> +       struct desc_ptr gdt;
>> +       int cpu = raw_smp_processor_id();
>> +       bool restore = false;
>> +       struct desc_struct *fixmap_gdt;
>> +
>> +       native_store_gdt(&gdt);
>
> Off the top of my head, this is something like 10 cycles.  IMO that's
> fast enough not to worry about the regression this will cause to KVM
> exits.  In any event, we'll get that back and *much* more when we do
> the optimizations that this series enables.



-- 
Thomas

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