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Message-ID: <33bcc5778e39780c6895ffa9f52f4b12cf83ad89.camel@infradead.org>
Subject: Re: [syzbot] [kvm?] WARNING in __kvm_gpc_refresh
From: David Woodhouse <dwmw2@infradead.org>
To: Sean Christopherson <seanjc@google.com>
Cc: syzbot <syzbot+106a4f72b0474e1d1b33@syzkaller.appspotmail.com>, 
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, 
	syzkaller-bugs@googlegroups.com, paul <paul@xen.org>
Date: Mon, 18 Mar 2024 21:55:47 +0000
In-Reply-To: <ZfizYzC9-9Qo47tE@google.com>
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	 <b7561e6d6d357fcd8ec1a1257aaf2f97d971061c.camel@infradead.org>
	 <ZfizYzC9-9Qo47tE@google.com>
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On Mon, 2024-03-18 at 14:34 -0700, Sean Christopherson wrote:
> On Mon, Mar 18, 2024, David Woodhouse wrote:
> >=20
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* Either gpa or uhva must b=
e valid, but not both */
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (WARN_ON_ONCE(kvm_is_erro=
r_gpa(gpa) =3D=3D kvm_is_error_hva(uhva)))
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 return -EINVAL;
> >=20
> > Hm, that comment doesn't match the code. It says "not both", but the
> > code also catches the "neither" case. I think the gpa is in %rbx and
> > uhva is in %r12, so this is indeed the 'neither' case.
> >=20
> > Is it expected that we can end up with a cache marked active, but with
> > the address not valid? Maybe through a race condition with deactive? or
> > more likely than that?
>=20
> It's the darn PV system time MSR, which allows the guest to triggering ac=
tivation
> with any GPA value.=C2=A0 That results in the cache being marked active w=
ithout KVM
> ever setting the GPA (or any other fields).=C2=A0 The fix I'm testing is =
to move the
> offset+len check up into activate() and refresh().

Not sure I even want a gpc of length 1 to work at INVALID_GPA; I don't
think it's the offset+length check we want to be looking at?

If we've activated the gpc with gpa=3D=3DINVALID_GPA, surely the right
thing to do is just let it fail (perhaps with an explicit check or just
letting the memslot lookup fail). After fixing that WARN_ON be

   if (WARN_ON_ONCE(!kvm_is_error_gpa(gpa) && !kvm_is_error_hva(uhva)))

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