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Subject: Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced
From: Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CA+55aFzYpsEj3_K0Ex_eWuvcb2WOZPwv5HrOA4k93vcayqganA@mail.gmail.com>
Date: Thu, 14 Dec 2017 14:30:34 -0800
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References: <20171214112726.742649793@infradead.org> <20171214113851.647809433@infradead.org> <CALCETrW0=FnqZMU_MLebyy5m7jj=w=yHYx=u6vghFkdmG7vsww@mail.gmail.com> <CA+55aFz71Ycm3oez30zOCztx1sio8ioy3VED2rE0ORoExXBz2g@mail.gmail.com> <CALCETrU8=z92_ZtwR9EO56eeOBE1LbxOqigZGO_yahmcM2dE_A@mail.gmail.com> <CA+55aFyNN4Lhf4RhL95oeGvfng=H4wKSA3-MwzMo=KpBocQ7bA@mail.gmail.com> <CA+55aFxmwpkDNT0YcaiG-BQ5SUT6h6YkevVfNkU_eY-F2E-h7Q@mail.gmail.com> <B3532E4F-408A-450A-96FF-17763916934C@amacapital.net> <CA+55aFzYpsEj3_K0Ex_eWuvcb2WOZPwv5HrOA4k93vcayqganA@mail.gmail.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>, Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bpetkov@suse.de>, Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>, Brian Gerst <brgerst@gmail.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Denys Vlasenko <dvlasenk@redhat.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Juergen Gross <jgross@suse.com>, David Laight <David.Laight@aculab.com>, Eduardo Valentin <eduval@amazon.com>, "Liguori, Anthony" <aliguori@amazon.com>, Will Deacon <will.deacon@arm.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Dan Williams <dan.j.williams@intel.com>



> On Dec 14, 2017, at 2:15 PM, Linus Torvalds <torvalds@linux-foundation.org=
> wrote:
>=20
>> On Thu, Dec 14, 2017 at 2:11 PM, Andy Lutomirski <luto@amacapital.net> wr=
ote:
>>=20
>> That seems to rather defeat the point of using a VMA, though.
>=20
> There never was any point in using a VMA per se.
>=20
> The point was always to just map the damn thing in the user page
> tables, wasn't it?
>=20
> The vma bit was just an implementation detail.

And all this is why I dislike using a VMA.  My patch puts it at a negative a=
ddress. We could just as easily put it just above TASK_SIZE_MAX, but I'm a b=
it nervous about bugs that overrun an access_ok check by a small amount.  II=
RC I found one of those in the net code once, and I didn't look very hard.

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