Subject: Re: [PATCH 1/5] KVM: rseq: Update rseq when processing NOTIFY_RESUME on xfer to KVM guest

From: Sean Christopherson <[email-masked]>
Date: Tue, 7 Sep 2021 14:38:31 +0000

On Mon, Sep 06, 2021, Paolo Bonzini wrote:
> On 20/08/21 20:51, Mathieu Desnoyers wrote:
> > > Ah, or is it the case that rseq_cs is non-NULL if and only if userspace is in an
> > > rseq critical section, and because syscalls in critical sections are illegal, by
> > > definition clearing rseq_cs is a nop unless userspace is misbehaving.
> > Not quite, as I described above. But we want it to stay set so the CONFIG_DEBUG_RSEQ
> > code executed when returning from ioctl to userspace will be able to validate that
> > it is not nested within a rseq critical section.
> > 
> > > If that's true, what about explicitly checking that at NOTIFY_RESUME?  Or is it
> > > not worth the extra code to detect an error that will likely be caught anyways?
> > The error will indeed already be caught on return from ioctl to userspace, so I
> > don't see any added value in duplicating this check.
> 
> Sean, can you send a v2 (even for this patch only would be okay)?

Made it all the way to v3 while you were out :-)

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