File CVE-2017-2620-xsa209-qemuu-cirrus_bitblt_cputovideo-does-not-check-if-memory-region-safe.patch of Package xen.7317

References: bsc#1024834 CVE-2017-2620 XSA-209

From: Gerd Hoffmann <kraxel@redhat.com>
Subject: [PATCH 3/3] cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo

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.

[ This is CVE-2017-2620 / XSA-209  - Ian Jackson ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/display/cirrus_vga.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Index: xen-4.4.4-testing/tools/qemu-xen-dir-remote/hw/display/cirrus_vga.c
===================================================================
--- xen-4.4.4-testing.orig/tools/qemu-xen-dir-remote/hw/display/cirrus_vga.c
+++ xen-4.4.4-testing/tools/qemu-xen-dir-remote/hw/display/cirrus_vga.c
@@ -862,6 +862,10 @@ static int cirrus_bitblt_cputovideo(Cirr
 {
     int w;
 
+    if (blit_is_unsafe(s)) {
+        return 0;
+    }
+
     s->cirrus_blt_mode &= ~CIRRUS_BLTMODE_MEMSYSSRC;
     s->cirrus_srcptr = &s->cirrus_bltbuf[0];
     s->cirrus_srcptr_end = &s->cirrus_bltbuf[0];
@@ -887,6 +891,10 @@ static int cirrus_bitblt_cputovideo(Cirr
 	}
         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);