Re: [PATCH 08/16] x86/shstk: Create shadow stacks

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/05/2020 15:55, Jan Beulich wrote:
>> +            /* Poision unused entries. */
>> +            for ( i = IST_MAX;
>> +                  i < ARRAY_SIZE(this_cpu(tss_page).ist_ssp); ++i )
>> +                    ist_ssp[i] = 0x8600111111111111ul;
> IST_MAX == IST_DF, so you're overwriting one token here.

And failing to poison entry 0.  This was a bad rearrangement when
tidying the series up.

Unfortunately, testing the #DF path isn't terribly easy.

>> --- a/xen/include/asm-x86/processor.h
>> +++ b/xen/include/asm-x86/processor.h
>> @@ -434,7 +434,8 @@ struct __packed tss64 {
>>      uint16_t :16, bitmap;
>>  };
>>  struct tss_page {
>> -    struct tss64 __aligned(PAGE_SIZE) tss;
>> +    uint64_t __aligned(PAGE_SIZE) ist_ssp[8];
>> +    struct tss64 tss;
>>  };
> Just curious - any particular reason you put this ahead of the TSS?

Yes.  Reduced chance of interacting with a buggy IO bitmap offset.

Furthermore, we could do away most of the IO emulation quirking, and the
#GP path overhead, if we actually constructed a real IO bitmap for
dom0.  That would require using the 8k following the TSS.

~Andrew