On Mon, Mar 7, 2016 at 10:22 AM, Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
> On 03/07/2016 11:08 AM, Andy Lutomirski wrote:
>>
>> On Mon, Mar 7, 2016 at 10:04 AM, Khalid Aziz <khalid.aziz@xxxxxxxxxx>
>> wrote:
>>>
>>> On 03/07/2016 09:56 AM, David Miller wrote:
>>>>
>>>>
>>>> From: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
>>>> Date: Mon, 7 Mar 2016 08:07:53 -0700
>>>>
>>>>> PR_GET_SPARC_ADICAPS
>>>>
>>>>
>>>>
>>>> Put this into a new ELF auxiliary vector entry via ARCH_DLINFO.
>>>>
>>>> So now all that's left is supposedly the TAG stuff, please explain
>>>> that to me so I can direct you to the correct existing interface to
>>>> provide that as well.
>>>>
>>>> Really, try to avoid prtctl, it's poorly typed and almost worse than
>>>> ioctl().
>>>>
>>>
>>> The two remaining operations I am looking at are:
>>>
>>> 1. Is PSTATE.mcde bit set for the process? PR_SET_SPARC_ADI provides this
>>> in
>>> its return value in the patch I sent.
>>>
>>> 2. Is TTE.mcd set for a given virtual address? PR_GET_SPARC_ADI_STATUS
>>> provides this function in the patch I sent.
>>>
>>> Setting and clearing version tags can be done entirely from userspace:
>>>
>>> while (addr < end) {
>>> asm volatile(
>>> "stxa %1, [%0]ASI_MCD_PRIMARY\n\t"
>>> :
>>> : "r" (addr), "r" (version));
>>> addr += adicap.blksz;
>>> }
>>> so I do not have to add any kernel code for tags.
>>
>>
>> Is the effect of that to change the tag associated with a page to
>> which the caller has write access?
>
>
> No, it changes the tag associated with the virtual address for the caller.
> Physical page backing this virtual address is unaffected. Tag checking is
> done for virtual addresses. The one restriction where physical address is
> relevant is when two processes map the same physical page, they both have to
> use the same tag for the virtual addresses that map on to the shared
> physical pages.
Slow down, please. *Why* do the tags for two different VAs that map
to the same PA have to match? What goes wrong if they don't, and why
is requiring them to be the same a good idea?
>
>>
>> I sense DoS issues in your future.
>>
>
> Are you concerned about DoS even if the tag is associated with virtual
> address, not physical address?
Yes, absolutely.
fd = open("/lib/ld.so");
mmap(fd)
stxa to write the tag
*boom*, presumably, because the tags apparently have to match for all mappings.
What data structure or structures changes when this stxa instruction happens?
--Andy
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