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

[Thomas Gleixner]

On Tue, Nov 17 2020 at 09:42, Alexandre Chartre wrote:
> On 11/17/20 12:06 AM, Andy Lutomirski wrote:
> 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

Well, the PTI stack might have guard pages, but the kernel stack can now
underflow into the PTI stack. Not good.

Thanks,

        tglx