[Qemu-stable] [PULL 1/1] cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo (CVE-2017-2620)

[Gerd Hoffmann]

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.

Cc: address@hidden
Signed-off-by: Gerd Hoffmann <address@hidden>
---
 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 1deb520..b9e7cb1 100644
--- a/hw/display/cirrus_vga.c
+++ b/hw/display/cirrus_vga.c
@@ -900,6 +900,10 @@ 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];
@@ -925,6 +929,10 @@ 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