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Subject: Re: [RFC PATCH 00/16] PTI support for x86-32
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CA+55aFz4cUhqhmWg-F8NXGjowVGXkMA126H-mQ4n1A0ywtQ_tg@mail.gmail.com>
Date:   Sun, 21 Jan 2018 18:27:59 -0800
Cc:     Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
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        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
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        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Brian Gerst <brgerst@gmail.com>,
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        Eduardo Valentin <eduval@amazon.com>,
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        Kees Cook <keescook@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Joerg Roedel <jroedel@suse.de>
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To:     Linus Torvalds <torvalds@linux-foundation.org>
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Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Sun, Jan 21, 2018 at 3:46 PM, Nadav Amit <nadav.amit@gmail.com> =
wrote:
>> I wanted to see whether segments protection can be a replacement for =
PTI
>> (yes, excluding SMEP emulation), or whether speculative execution =
=E2=80=9Cignores=E2=80=9D
>> limit checks, similarly to the way paging protection is skipped.
>>=20
>> It does seem that segmentation provides sufficient protection from =
Meltdown.
>> The =E2=80=9Creliability=E2=80=9D test of Gratz PoC fails if the =
segment limit is set to
>> prevent access to the kernel memory. [ It passes if the limit is not =
set,
>> even if the DS is reloaded. ] My test is enclosed below.
>=20
> Interesting. It might not be entirely reliable for all
> microarchitectures, though.
>=20
>> So my question: wouldn=E2=80=99t it be much more efficient to use =
segmentation
>> protection for x86-32, and allow users to choose whether they want =
SMEP-like
>> protection if needed (and then enable PTI)?
>=20
> That's what we did long long ago, with user space segments actually
> using the limit (in fact, if you go back far enough, the kernel even
> used the base).
>=20
> You'd have to make sure that the LDT loading etc do not allow CPL3
> segments with base+limit past TASK_SIZE, so that people can't generate
> their own.  And the TLS segments also need to be limited (and
> remember, the limit has to be TASK_SIZE-base, not just TASK_SIZE).
>=20
> And we should check with Intel that segment limit checking really is
> guaranteed to be done before any access.

Thanks. I=E2=80=99ll try to check with Intel liaison people of VMware =
(my employer),
yet any feedback will be appreciated.

> Too bad x86-64 got rid of the segments ;)

Actually, as I noted in a different thread, running 32-bit binaries on
x86_64 in legacy-mode, without PTI, performs considerably better than =
x86_64
binaries with PTI for workloads that are hit the most (e.g., Redis). By
dynamically removing the 64-bit user-CS from the GDT, this mode should =
be
safe, as long as CS load is not done speculatively.

Regards,
Nadav=