Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame Ownership

[Khalid Aziz]

On 1/10/19 5:44 PM, Andy Lutomirski wrote:
> On Thu, Jan 10, 2019 at 3:07 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>
>> On Thu, Jan 10, 2019 at 1:10 PM Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
>>> I implemented a solution to reduce performance penalty and
>>> that has had large impact. When XPFO code flushes stale TLB entries,
>>> it does so for all CPUs on the system which may include CPUs that
>>> may not have any matching TLB entries or may never be scheduled to
>>> run the userspace task causing TLB flush. Problem is made worse by
>>> the fact that if number of entries being flushed exceeds
>>> tlb_single_page_flush_ceiling, it results in a full TLB flush on
>>> every CPU. A rogue process can launch a ret2dir attack only from a
>>> CPU that has dual mapping for its pages in physmap in its TLB. We
>>> can hence defer TLB flush on a CPU until a process that would have
>>> caused a TLB flush is scheduled on that CPU. I have added a cpumask
>>> to task_struct which is then used to post pending TLB flush on CPUs
>>> other than the one a process is running on. This cpumask is checked
>>> when a process migrates to a new CPU and TLB is flushed at that
>>> time. I measured system time for parallel make with unmodified 4.20
>>> kernel, 4.20 with XPFO patches before this optimization and then
>>> again after applying this optimization. Here are the results:
> 
> I wasn't cc'd on the patch, so I don't know the exact details.
> 
> I'm assuming that "ret2dir" means that you corrupt the kernel into
> using a direct-map page as its stack.  If so, then I don't see why the
> task in whose context the attack is launched needs to be the same
> process as the one that has the page mapped for user access.

You are right. More work is needed to refine delayed TLB flush to close
this gap.

> 
> My advice would be to attempt an entirely different optimization: try
> to avoid putting pages *back* into the direct map when they're freed
> until there is an actual need to use them for kernel purposes.

I had thought about that but it turns out the performance impact happens
on the initial allocation of the page and resulting TLB flushes, not
from putting the pages back into direct map. The way we could benefit
from not adding page back to direct map is if we change page allocation
to prefer pages not in direct map. That way we incur the cost of TLB
flushes initially but then satisfy multiple allocation requests after
that from those "xpfo cost" free pages. More changes will be needed to
pick which of these pages can be added back to direct map without
degenerating into worst case scenario of a page bouncing constantly
between this list of preferred pages and direct mapped pages. It started
to get complex enough that I decided to put this in my back pocket and
attempt simpler approaches first :)

> 
> How are you handing page cache?  Presumably MAP_SHARED PROT_WRITE
> pages are still in the direct map so that IO works.
> 

Since Juerg wrote the actual implementation of XPFO, he probably
understands it better. XPFO tackles only the page allocation requests
from userspace and does not touch page cache pages.

--
Khalid

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