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

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Date:   Tue, 21 Nov 2017 15:17:15 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...capital.net>
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 11/21/2017 02:46 PM, Andy Lutomirski wrote:
>> GDT: R/O TSS: R/W at least because of trampoline stack entry code:
>> EXEC+R/O exception stacks: R/W
> Can you avoid code duplication by adding some logic right after the
> kernel cpu_entry_area is set up to iterate page by page over the PTEs
> in the cpu_entry_area for that CPU and just install exactly the same
> PTEs into the kaiser table?  E.g. just call kaiser_add_mapping once
> per page but with the parameters read out from the fixmap PTEs
> instead of hard coded?

Yes, we could do that.  But, what's the gain?  We end up removing
effectively three (long) lines of code from three kaiser_add_mapping()
calls.

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 guess the upside is that we don't open-code the permissions in the
KAISER code that *have* to match the permissions that the kernel itself
established.

It also means that theoretically you could not touch the KAISER code the
next time we expand the cpu entry area.