Re: [RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings

[Alexandre Chartre]

On 11/17/20 12:06 AM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 12:18 PM Alexandre Chartre
> <alexandre.chartre@xxxxxxxxxx> wrote:
> > On 11/16/20 8:48 PM, Andy Lutomirski wrote:
> > > On Mon, Nov 16, 2020 at 6:49 AM Alexandre Chartre
> > > <alexandre.chartre@xxxxxxxxxx> wrote:
> > > > Extend PTI user mappings so that more kernel entry code can be executed
> > > > with the user page-table. To do so, we need to map syscall and interrupt
> > > > entry code, per cpu offsets (__per_cpu_offset, which is used some in
> > > > entry code), the stack canary, and the PTI stack (which is defined per
> > > > task).
> > >
> > > Does anything unmap the PTI stack?  Mapping is easy, and unmapping
> > > could be a pretty big mess.
> >
> > No, there's no unmap. The mapping exists as long as the task page-table
> > does (i.e. as long as the task mm exits). I assume that the task stack
> > and mm are freed at the same time but that's not something I have checked.
>
> Nope.  A multi-threaded mm will free task stacks when the task exits,
> but the mm may outlive the individual tasks.  Additionally, if you
> allocate page tables as part of mapping PTI stacks, you need to make
> sure the pagetables are freed.

So I think I just need to unmap the PTI stack from the user page-table
when the task exits. Everything else is handled because the kernel and
PTI stack are allocated in a single chunk (referenced by task->stack).


>  Finally, you need to make sure that
> the PTI stacks have appropriate guard pages -- just doubling the
> allocation is not safe enough.

The PTI stack does have guard pages because it maps only a part of the task
stack into the user page-table, so pages around the PTI stack are not mapped
into the user-pagetable (the page below is the task stack guard, and the page
above is part of the kernel-only stack so it's never mapped into the user
page-table).

+ *   +-------------+
+ *   |             | ^                       ^
+ *   | kernel-only | | KERNEL_STACK_SIZE     |
+ *   |    stack    | |                       |
+ *   |             | V                       |
+ *   +-------------+ <- top of kernel stack  | THREAD_SIZE
+ *   |             | ^                       |
+ *   | kernel and  | | KERNEL_STACK_SIZE     |
+ *   | PTI stack   | |                       |
+ *   |             | V                       v
+ *   +-------------+ <- top of stack

> My intuition is that this is going to be far more complexity than is justified.

Sounds like only the PTI stack unmap is missing, which is hopefully not
that bad. I will check that.

alex.