From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D3B0244102B; Tue, 28 Apr 2026 14:54:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777388082; cv=none; b=IeWpg0IwGszpXY2C7lohPxxRXp9zZqTzGBpqBFnCmhwTd9w1zh/4SCEr3ppPuAb03JAJKUnVL5qEm/syZdYMktXRobtWek7gSkuVGgD8DMPr9Cs/U0Cjo2HrN+HgeMv5W9I4ZN+wAhzdlnO+5vBtEA3Uxily5hD1RUXTxl+Rilg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777388082; c=relaxed/simple; bh=qcw1/DuQoLk7cCrzWZwg5WEvXeuuf2AKc5s4H51c4WQ=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=N3uwHvK/gtFFk71KeZdFbUEC/QSHYnBBvLuqJILe+++Fs0PfgZhzjKnNV0Ta+OpLJyw5OJtWDsKnBrxA1wv0OVXm6NouBxhacIcbYQB2O+92BE+q/jJLq7tiG2NqHE5P/fa4oVrvsM+8KFebhkw3Ur/xoDe5Y8GvR+PGipo4/eY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=desiato.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=E8W3mchc; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=desiato.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="E8W3mchc" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=qcw1/DuQoLk7cCrzWZwg5WEvXeuuf2AKc5s4H51c4WQ=; b=E8W3mchcRcObh60YgYvSPRT/bF 6ySkQ4yfhyJlr0ID+dUEmQOU73FGTeicIyAkpLZWhfL6HTSjJtbXVxuNaOJeaPLKokUUI6Nw/z0df PcnC3y0wkWoWKFY3c8fx+9au1tuxG+Pl+KMpEHFFeeGlyVzoUsMIPe9rKPd5mCsD+8TPPeklDCPaN bv/KKrFa8bqU6PTMk3GFpn4woveONVxllognHWHSPh9yTCYPgwgGmIC1Yf7svLNJe6umluyQn8Utq H1HV8aiZJqQ1QVRGCegb5BFQXJ2lEp59EDfHTSGAlU9MnBZx5EDnPvEG2WtnQjM/w40g6T6t9fZWK aeoh96rQ==; Received: from [172.31.31.148] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1wHjpZ-00000003Oar-1p7O; Tue, 28 Apr 2026 14:54:34 +0000 Message-ID: Subject: Re: [PATCH 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range From: David Woodhouse To: Sean Christopherson Cc: Paolo Bonzini , Paul Durrant , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+cdeaeec70992eca2d920@syzkaller.appspotmail.com, Joao Martins Date: Tue, 28 Apr 2026 15:54:33 +0100 In-Reply-To: References: <20250201011400.669483-1-seanjc@google.com> <20250201011400.669483-2-seanjc@google.com> <43f702b383fb99d435f2cdb8ef35cc1449fe6c23.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-3m+frVvsqM/hG5Lc+xiv" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html --=-3m+frVvsqM/hG5Lc+xiv Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2025-02-05 at 15:26 +0000, David Woodhouse wrote: >=20 > > > @@ -3733,7 +3733,13 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, = struct msr_data *msr_info) > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 u32 msr =3D msr_info->inde= x; > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 u64 data =3D msr_info->dat= a; > > > =C2=A0 > > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (msr && msr =3D=3D vcpu->kvm= ->arch.xen_hvm_config.msr) > > > +=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 * Do not allow host-initi= ated writes to trigger the Xen hypercall > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * page setup; it could in= cur locking paths which are not expected > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * if userspace sets the M= SR in an unusual location. > >=20 > > That's just as likely to break userspace.=C2=A0 Doing a save/restore on= the MSR doesn't > > make a whole lot of sense since it's effectively a "command" MSR, but I= MO it's not > > any less likely than userspace putting the MSR index outside of the syn= thetic range. >=20 > Save/restore on the MSR makes no sense. It's a write-only MSR; writing > to it has no effect *other* than populating the target page. In KVM we > don't implement reading from it at all; I don't think Xen does either? >=20 > And even if it was readable and would rather pointlessly return the > last value written to it, save/restore arguably shouldn't actually > trigger the guest memory to be overwritten again. The hypercall page > should only be populated when the *guest* writes the MSR. >=20 > With the recent elimination of the hypercall page from Linux Xen > guests, we've suggested that Linux should still set up the hypercall > page early (as it *does* have the side-effect of letting Xen know that > the guest is 64-bit). And then just free the page without ever using > it. We absolutely would not want a save/restore to scribble on that > page again. >=20 > I'm absolutely not worried about breaking userspace with such a change > to make the hypercall page MSR only work when !host_initiated. In fact > I think it's probably the right thing to do *anyway*. >=20 > If userspace wants to write to guest memory, it can do that anyway; it > doesn't need to ask the *kernel* to do it. In the meantime, our VMM actually did start to intercept the MSR write and then 'replay' it to the kernel with ->host_initiated. It does so because the MSR writes correlate with kexec/crash in the guest, and tracking that allows the VMM to deal with resetting e.g. the blkback/netback rings in order that the crash kernel can actually complete the crashdump. I pondered reverting commit 3617c0ee7dec but I do still think it makes sense for all the reasons we originally discussed. I pondered making VMM do everything for itself which kvm_xen_write_hypercall_page() does, but that's a bit icky. Settled for adding a KVM_XEN_HVM_CONFIG_WRITE_HYPERCALL_PAGE attribute which basically does the same operation directly, instead of triggering it through an MSR write. 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