linux-kernel - Re: [PATCH v2 11/17] selftests/x86/ldt

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Date:   Thu, 14 Dec 2017 14:11:03 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Will Deacon <will.deacon@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced


> On Dec 14, 2017, at 1:48 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Thu, Dec 14, 2017 at 1:44 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> 
>> So it clearly needs to have the PAGE_USER bit clear (to avoid users
>> accessing it directly), and it needs to be marked somehow for
>> get_user_pages() to refuse it too, and access_ok() needs to fail it so
>> that we can't do get_user/put_user on it.
> 
> Actually, just clearing PAGE_USER should make gup avoid it automatically.
> 
> So really the only other thing it needs is to have access_ok() avoid
> it so that the kernel can't be fooled into accessing it for the user.
> 
> That does probably mean having to put it at the top of the user
> address space and playing games with user_addr_max(). Which is not
> wonderful, but certainly not rocket surgery either.

That seems to rather defeat the point of using a VMA, though.  And it means we still have to do a full cmp instead of just checking a sign bit in access_ok if we ever manage to kill set_fs().

Again, I have an apparently fully functional patch to alias the LDT at a high (kernel) address where we can cleanly map it in the user pagetables without any of this VMA stuff.  It's much less code.