linux-kernel - Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

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Date:   Tue, 21 Nov 2017 16:17:02 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

On Tue, Nov 21, 2017 at 3:42 PM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> On 11/21/2017 03:32 PM, Andy Lutomirski wrote:
>>> To do this, we need to special-case the kernel page table walker to deal
>>> with PTEs only since we can't just grab PMD or PUD flags and stick them
>>> in a PTE.  We would only be able to use this path when populating things
>>> that we know are 4k-mapped in the kernel.
>> I'm not sure I'm understanding the issue.  We'd promise to map the
>> cpu_entry_area without using large pages, but I'm not sure I know what
>> you're referring to.  The only issue I see is that we'd have to be
>> quite careful when tearing down the user tables to avoid freeing the
>> shared part.
>
> It's just that it currently handles large and small pages in the kernel
> mapping that it's copying.  If we want to have it just copy the PTE,
> we've got to refactor things a bit to separate out the PTE flags from
> the paddr being targeted, and also make sure we don't munge the flags
> conversion from the large-page entries to 4k PTEs.  The PAT and PSE bits
> cause a bit of trouble here.

I'm confused.  I mean something like:

unsigned long start = (unsigned long)get_cpu_entry_area(cpu);
for (unsigned long addr = start; addr < start + sizeof(struct
cpu_entry_area); addr += PAGE_SIZE) {
  pte_t pte = *pte_offset_k(addr);  /* or however you do this */
  kaiser_add_mapping(pte_pfn(pte), pte_prot(pte));
}

modulo the huge pile of typos in there that surely exist.

But I still prefer my approach of just sharing the cpu_entry_area pmd
entries between the user and kernel tables.