From 15268f91fbe75b38a851c458aef74e693d646ea5 Mon Sep 17 00:00:00 2001

From: Gerd Hoffmann <kraxel@redhat.com>

Date: Tue, 21 Feb 2017 10:54:59 -0800

Subject: [PATCH 2/2] 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 ]

Reported-by: Gerd Hoffmann <ghoffman@redhat.com>

Signed-off-by: Gerd Hoffmann <kraxel@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 34a6900..5901250 100644

--- a/hw/display/cirrus_vga.c

+++ b/hw/display/cirrus_vga.c

@@ -865,6 +865,10 @@ 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];

@@ -890,6 +894,10 @@ 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);

-- 

2.1.4