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From 74db251c34369bd32148864b3abea6d6586270dc Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Fri, 10 Feb 2017 08:30:15 +0100
Subject: [PATCH 3/3] cirrus: add blit_is_unsafe call to
 cirrus_bitblt_cputovideo

RH-Author: Gerd Hoffmann <kraxel@redhat.com>
Message-id: <1486715415-3462-4-git-send-email-kraxel@redhat.com>
Patchwork-id: 73773
O-Subject: [virt-devel] [EMBARGOED RHEL-7.4 qemu-kvm PATCH 3/3] cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo
Bugzilla: 1420492
CVE: CVE-2017-2620/20170221
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: Laurent Vivier <lvivier@redhat.com>
RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

CIRRUS_BLTMODE_MEMSYSSRC blits do NOT check blit destination
and blit width, at all.  Oops.  Fix it.

Security impact: high.

The missing blit destination check allows to write to host memory.
Basically same as CVE-2014-8106 for the other blit variants.

The missing blit width check allows to overflow cirrus_bltbuf,
with the attractive target cirrus_srcptr (current cirrus_bltbuf write
position) being located right after cirrus_bltbuf in CirrusVGAState.

Due to cirrus emulation writing cirrus_bltbuf bytewise the attacker
hasn't full control over cirrus_srcptr though, only one byte can be
changed.  Once the first byte has been modified further writes land
elsewhere.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Upstream: TBD (embargoed until Feb 21st)
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 hw/display/cirrus_vga.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
index 9a9501f..1b972db 100644
--- a/hw/display/cirrus_vga.c
+++ b/hw/display/cirrus_vga.c
@@ -894,6 +894,10 @@ static int cirrus_bitblt_cputovideo(CirrusVGAState * s)
 {
     int w;
 
+    if (blit_is_unsafe(s, true)) {
+        return 0;
+    }
+
     s->cirrus_blt_mode &= ~CIRRUS_BLTMODE_MEMSYSSRC;
     s->cirrus_srcptr = &s->cirrus_bltbuf[0];
     s->cirrus_srcptr_end = &s->cirrus_bltbuf[0];
@@ -919,6 +923,10 @@ static int cirrus_bitblt_cputovideo(CirrusVGAState * s)
 	}
         s->cirrus_srccounter = s->cirrus_blt_srcpitch * s->cirrus_blt_height;
     }
+
+    /* the blit_is_unsafe call above should catch this */
+    assert(s->cirrus_blt_srcpitch <= CIRRUS_BLTBUFSIZE);
+
     s->cirrus_srcptr = s->cirrus_bltbuf;
     s->cirrus_srcptr_end = s->cirrus_bltbuf + s->cirrus_blt_srcpitch;
     cirrus_update_memory_access(s);
-- 
1.8.3.1