linux-kernel - Re: [patch 01/10] x86/fpu/signal: Clarify exception handling in restore_fpregs_from

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Date:   Thu, 02 Sep 2021 16:08:11 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [patch 01/10] x86/fpu/signal: Clarify exception handling in
 restore_fpregs_from_user()

On Wed, 2021-09-01 at 16:47 +0000, Sean Christopherson wrote:
> On Wed, Sep 01, 2021, Thomas Gleixner wrote:
> > On Wed, Sep 01 2021 at 14:00, Thomas Gleixner wrote:
> > > commit b2f9d678e28c ("x86/mce: Check for faults tagged in
> > > EXTABLE_CLASS_FAULT exception table entries") made use of this in MCE to
> > > allow in kernel recovery. The only thing it uses is checking the
> > > exception handler type.
> > > 
> > > Bah. I'll fix that up to make that less obscure.
> > > 
> > > The remaining two use cases (SGX and FPU) make use of the stored trap
> > > number.
> > 
> > Though while for the FPU use case we really want to handle the #MC case,
> > it's not clear to me whether this is actually correct for SGX.
> > 
> > Jarkko, Sean, Dave?
> 
> Are you asking about #MC specifically, or about SGX consuming the trap number in
> general?
> 
> For #MC, it's probably a moot point because #MC on ENCLS is not recoverable for
> current hardware.  If #MC somehow occurs on ENCLS and doesn't kill the platform,
> "handling" the #MC in SGX is probably wrong.  Note, Tony is working on a series to
> support recoverable #MC on SGX stuff on future hardware[*], but I'm not sure that's
> relevant to this discussion.
> 
> As for SGX consuming the trap number in general, it's correct.  For non-KVM usage,
> it's nice to have but not strictly necessary.  Any fault except #PF on ENCLS is
> guaranteed to be a kernel or hardware bug; SGX uses the trap number to WARN on a
> !#PF exception, e.g. on #GP or #UD.  Not having the trap number would mean losing
> those sanity checks, which have been useful in the past.

AFAIK, we do not consider #UD as a bug. Agree with the conclusion that SGX
should never #MC, I just did not get this part. #UD is something that is
useful for SGX run-time.

/Jarkko