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    SubjectRe: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code
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    On 11/17/20 6:07 PM, Borislav Petkov wrote:
    > On Tue, Nov 17, 2020 at 09:19:01AM +0100, Alexandre Chartre wrote:
    >> We are not reversing PTI, we are extending it.
    >
    > You're reversing it in the sense that you're mapping more kernel memory
    > into the user page table than what is mapped now.
    >
    >> PTI removes all kernel mapping from the user page-table. However there's
    >> no issue with mapping some kernel data into the user page-table as long as
    >> these data have no sensitive information.
    >
    > I hope that is the case.
    >
    >> Actually, PTI is already doing that but with a very limited scope. PTI adds
    >> into the user page-table some kernel mappings which are needed for userland
    >> to enter the kernel (such as the kernel entry text, the ESPFIX, the
    >> CPU_ENTRY_AREA_BASE...).
    >>
    >> So here, we are extending the PTI mapping so that we can execute more kernel
    >> code while using the user page-table; it's a kind of PTI on steroids.
    >
    > And this is what bothers me - someone else might come after you and say,
    > but but, I need to map more stuff into the user pgt because I wanna do
    > X... and so on.

    Agree, any addition should be strictly checked. I have been careful to expand
    it to the minimum I needed.


    >> The minimum size would be 1 page (4KB) as this is the minimum mapping size.
    >> It's certainly enough for now as the usage of the PTI stack is limited, but
    >> we will need larger stack if we won't to execute more kernel code with the
    >> user page-table.
    >
    > So on a big machine with a million tasks, that's at least a million
    > pages more which is what, ~4 Gb?
    >
    > There better be a very good justification for the additional memory
    > consumption...

    Yeah, adding a per-task allocation is my main concern, hence this RFC.


    alex.

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