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Message-ID: <1516565047.9814.59.camel@infradead.org>
Subject: Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB
From:   David Woodhouse <dwmw2@infradead.org>
To:     Andrew Cooper <andrew.cooper3@citrix.com>,
        Borislav Petkov <bp@alien8.de>,
        KarimAllah Ahmed <karahmed@amazon.com>
Cc:     arjan@linux.intel.com, tglx@linutronix.de, karahmed@amazon.de,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        tim.c.chen@linux.intel.com, peterz@infradead.org,
        pbonzini@redhat.com, ak@linux.intel.com,
        torvalds@linux-foundation.org, gregkh@linux-foundation.org
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Date:   Sun, 21 Jan 2018 20:04:07 +0000
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On Sun, 2018-01-21 at 19:37 +0000, Andrew Cooper wrote:
>=20
> It doesn't matter if an attacker can use SP1 to try and skip the IBPB.
>=20
> Exits to userspace/guest are serialising (with some retroactive updates
> to the architecture spec coming), so an attacker can't cause victim code
> to be executed before speculation has caught up and noticed that the
> IBPB did need to happen.

For the specific case of IBPB, knowing what we do about non-
architectural behaviour, that's probably true.

In the early patch sets in both Xen and Linux, we did have a
conditional branch on {sys,hyper}call entry that blithely let the CPU
speculate all the way to the {sys,hyper}call table jump. No exit to
userspace/guest there.

Which is why I've been saying I want call sites to have an *explicit*
comment saying why they're safe to use conditional branches without
taking extra steps to be safe, like the 'else lfence'. And why I'd
really like the underlying primitives to *support* being fixed at
runtime.

ALTERNATIVE is fine for now, and can end up with basically the same
code as static_cpu_has() =E2=80=94 either we do the wrmsr, or we jump/nop o=
ver
where it used to be. Let's worry about getting clever with it *later*.
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