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Message-ID: <20170223094353.hrdgcxjsqdoy3v5l@dhcp-3-221.uk.xensource.com>
Date: Thu, 23 Feb 2017 09:43:53 +0000
From: Roger Pau Monné <roger.pau@...rix.com>
To: Xen.org security team <security@....org>
CC: <xen-announce@...ts.xen.org>, <xen-devel@...ts.xen.org>,
<xen-users@...ts.xen.org>, <oss-security@...ts.openwall.com>
Subject: Re: [Xen-devel] Xen Security Advisory 209 (CVE-2017-2620) -
cirrus_bitblt_cputovideo does not check if memory region is safe
On Tue, Feb 21, 2017 at 12:00:03PM +0000, Xen.org security team wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Xen Security Advisory CVE-2017-2620 / XSA-209
> version 3
>
> cirrus_bitblt_cputovideo does not check if memory region is safe
>
> UPDATES IN VERSION 3
> ====================
>
> Public release.
>
> ISSUE DESCRIPTION
> =================
>
> In CIRRUS_BLTMODE_MEMSYSSRC mode the bitblit copy routine
> cirrus_bitblt_cputovideo fails to check wethehr the specified memory
> region is safe.
>
> IMPACT
> ======
>
> A malicious guest administrator can cause an out of bounds memory
> write, very likely exploitable as a privilege escalation.
>
> VULNERABLE SYSTEMS
> ==================
>
> Versions of qemu shipped with all Xen versions are vulnerable.
>
> Xen systems running on x86 with HVM guests, with the qemu process
> running in dom0 are vulnerable.
>
> Only guests provided with the "cirrus" emulated video card can exploit
> the vulnerability. The non-default "stdvga" emulated video card is
> not vulnerable. (With xl the emulated video card is controlled by the
> "stdvga=" and "vga=" domain configuration options.)
>
> ARM systems are not vulnerable. Systems using only PV guests are not
> vulnerable.
>
> For VMs whose qemu process is running in a stub domain, a successful
> attacker will only gain the privileges of that stubdom, which should
> be only over the guest itself.
>
> Both upstream-based versions of qemu (device_model_version="qemu-xen")
> and `traditional' qemu (device_model_version="qemu-xen-traditional")
> are vulnerable.
>
> MITIGATION
> ==========
>
> Running only PV guests will avoid the issue.
>
> Running HVM guests with the device model in a stubdomain will mitigate
> the issue.
>
> Changing the video card emulation to stdvga (stdvga=1, vga="stdvga",
> in the xl domain configuration) will avoid the vulnerability.
>
> CREDITS
> =======
>
> This issue was discovered by Gerd Hoffmann of Red Hat.
>
> RESOLUTION
> ==========
>
> Applying the appropriate attached patch resolves this issue.
>
> xsa209-qemuu.patch qemu-xen, qemu upstream
> (no backport yet) qemu-xen-traditional
It would be nice to mention that (at least on QEMU shipped with 4.7) the
following patch is also needed for the XSA-209 fix to build correctly:
52b7f43c8fa185ab856bcaacda7abc9a6fc07f84
display: cirrus: ignore source pitch value as needed in blit_is_unsafe
Roger.
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