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Message-ID: <1516482364.9814.18.camel@amazon.co.uk>
Subject: Re: [RFC 04/10] x86/mm: Only flush indirect branches when
 switching into non dumpable process
To:     KarimAllah Ahmed <karahmed@amazon.de>,
        <linux-kernel@vger.kernel.org>
CC:     Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        "Dave Hansen" <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, <kvm@vger.kernel.org>,
        <x86@kernel.org>
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         <1516476182-5153-5-git-send-email-karahmed@amazon.de>
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Date:   Sat, 20 Jan 2018 21:06:04 +0000
MIME-Version: 1.0
From:   "Woodhouse, David" <dwmw@amazon.co.uk>
Sender: linux-kernel-owner@vger.kernel.org
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On Sat, 2018-01-20 at 20:22 +0100, KarimAllah Ahmed wrote:
> From: Tim Chen <tim.c.chen@linux.intel.com>

I think this is probably From: Andi now rather than From: Tim?

We do need the series this far in order to have a full retpoline-based
mitigation, and I'd like to see that go in sooner rather than later.
There's a little more discussion to be had about the IBRS parts which
come later in the series (and the final one or two which weren't posted
yet).

I think this is the one patch of the "we want this now" IBPB set that
we expect serious debate on, which is why it's still a separate
"optimisation" patch on top of the previous one which just does IBPB
unconditionally.


> Flush indirect branches when switching into a process that marked
> itself non dumpable.=C2=A0=C2=A0This protects high value processes like g=
pg
> better, without having too high performance overhead.
>=20
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> ---
> =C2=A0arch/x86/mm/tlb.c | 13 ++++++++++++-
> =C2=A01 file changed, 12 insertions(+), 1 deletion(-)
>=20
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 304de7d..f64e80c 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -225,8 +225,19 @@ void switch_mm_irqs_off(struct mm_struct *prev, stru=
ct mm_struct *next,
> =C2=A0		=C2=A0* Avoid user/user BTB poisoning by flushing the branch pred=
ictor
> =C2=A0		=C2=A0* when switching between processes. This stops one process =
from
> =C2=A0		=C2=A0* doing Spectre-v2 attacks on another.
> +		=C2=A0*
> +		=C2=A0* As an optimization: Flush indirect branches only when
> +		=C2=A0* switching into processes that disable dumping.
> +		=C2=A0*
> +		=C2=A0* This will not flush when switching into kernel threads.
> +		=C2=A0* But it would flush when switching into idle and back
> +		=C2=A0*
> +		=C2=A0* It might be useful to have a one-off cache here
> +		=C2=A0* to also not flush the idle case, but we would need some
> +		=C2=A0* kind of stable sequence number to remember the previous mm.
> =C2=A0		=C2=A0*/
> -		indirect_branch_prediction_barrier();
> +		if (tsk && tsk->mm && get_dumpable(tsk->mm) !=3D SUID_DUMP_USER)
> +			indirect_branch_prediction_barrier();
> =C2=A0
> =C2=A0		if (IS_ENABLED(CONFIG_VMAP_STACK)) {
> =C2=A0			/*
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