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Message-ID: <dce1e4ce98446660905398e9a4c2afc49596a34c.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: Tue, 19 Mar 2024 15:57:30 +0000
In-Reply-To: <Zfmt8rxlF1ag1iA_@google.com>
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On Tue, 2024-03-19 at 08:23 -0700, Sean Christopherson wrote:
> On Mon, Mar 18, 2024, David Woodhouse wrote:
> > 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 mu=
st be valid, but not both */
> > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (WARN_ON_ONCE(kvm_is_=
error_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 th=
e
> > > > code also catches the "neither" case. I think the gpa is in %rbx an=
d
> > > > 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 w=
ith
> > > > 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 triggerin=
g activation
> > > with any GPA value.=C2=A0 That results in the cache being marked acti=
ve without 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().
> >=20
> > 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?
> >=20
> > If we've activated the gpc with gpa=3D=3DINVALID_GPA, surely the right
>=20
> This particular issue isn't due to activating with gpa=3D=3DINVALID_GPA, =
it's due to
> marking the gpc as active without actually activating it.=C2=A0 The offse=
t+length
> check is simply what causes KVM to prematurely bail from activation.

Ah, right. Yes, that makes more sense now; thanks.

> > 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
> >=20
> > =C2=A0=C2=A0 if (WARN_ON_ONCE(!kvm_is_error_gpa(gpa) && !kvm_is_error_h=
va(uhva)))
>=20
> I really don't want to relax the sanity check, as I feel strongly that KV=
M needs
> an invariant that an active cache is either GPA-based or HVA-based, i.e. =
that at
> least one of GPA or HVA is "valid".=C2=A0 In quotes because the GPA doesn=
't need to
> be fully validated, just something that doesn't trip kvm_is_error_gpa().

Agreed.

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