linux-kernel - Re: [PATCH v2 1/6] arm64: errata: Assume that unknown CPUs _are

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Message-ID: <CAD=FV=WnbMegiKDuV1HeWrGyQx42+bPZjwKQ09Q++b-fFBm7gg@mail.gmail.com>
Date: Thu, 19 Dec 2024 10:36:16 -0800
From: Doug Anderson <dianders@...omium.org>
To: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Mark Rutland <mark.rutland@....com>, 
	linux-arm-msm@...r.kernel.org, Jeffrey Hugo <quic_jhugo@...cinc.com>, 
	Julius Werner <jwerner@...omium.org>, linux-arm-kernel@...ts.infradead.org, 
	Roxana Bradescu <roxabee@...gle.com>, Trilok Soni <quic_tsoni@...cinc.com>, 
	bjorn.andersson@....qualcomm.com, stable@...r.kernel.org, 
	James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] arm64: errata: Assume that unknown CPUs _are_
 vulnerable to Spectre BHB

Hi,

On Thu, Dec 19, 2024 at 9:51 AM Will Deacon <will@...nel.org> wrote:
>
> On Fri, Dec 13, 2024 at 04:52:02PM -0800, Douglas Anderson wrote:
> > The code for detecting CPUs that are vulnerable to Spectre BHB was
> > based on a hardcoded list of CPU IDs that were known to be affected.
> > Unfortunately, the list mostly only contained the IDs of standard ARM
> > cores. The IDs for many cores that are minor variants of the standard
> > ARM cores (like many Qualcomm Kyro CPUs) weren't listed. This led the
> > code to assume that those variants were not affected.
> >
> > Flip the code on its head and instead list CPU IDs for cores that are
> > known to be _not_ affected. Now CPUs will be assumed vulnerable until
> > added to the list saying that they're safe.
> >
> > As of right now, the only CPU IDs added to the "unaffected" list are
> > ARM Cortex A35, A53, and A55. This list was created by looking at
> > older cores listed in cputype.h that weren't listed in the "affected"
> > list previously.
>
> There's a list of affected CPUs from Arm here:
>
> https://developer.arm.com/Arm%20Security%20Center/Spectre-BHB
>
> (obviously only covers their own designs).
>
> So it looks like A510 and A520 should be unaffected too, although I
> didn't check exhaustively. It also looks like A715 is affected but the
> whitepaper doesn't tell you what version of 'k' to use...
>
> > Unfortunately, while this solution is better than what we had before,
> > it's still an imperfect solution. Specifically there are two ways to
> > mitigate Spectre BHB and one of those ways is parameterized with a "k"
> > value indicating how many loops are needed to mitigate. If we have an
> > unknown CPU ID then we've got to guess about how to mitigate it. Since
> > more cores seem to be mitigated by looping (and because it's unlikely
> > that the needed FW code will be in place for FW mitigation for unknown
> > cores), we'll choose looping for unknown CPUs and choose the highest
> > "k" value of 32.
>
> I don't think we should guess. Just say vulnerable.

Ah, I see. So the series won't actually _fix_ anyone, it will just
properly report that we're vulnerable. I guess that works.


> > The downside of our guessing is that some CPUs may now report as
> > "mitigated" when in reality they should need a firmware mitigation.
> > We'll choose to put a WARN_ON splat in the logs in this case any time
> > we had to make a guess since guessing the right mitigation is pretty
> > awful. Hopefully this will encourage CPU vendors to add their CPU IDs
> > to the list.
>
> Hmm. We shouldn't have to guess here as the firmware mitigation is
> discoverable. So if it's unavailable and we're running an a CPU which
> needs it, then we're vulnerable.

Sure.

-Doug