[Xen-changelog] [qemu-xen-traditional stable-4.8] cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo

[patchbot@xxxxxxx]

commit b77d58b59ff0688e4aeb933450d07716ca0fb1f2
Author:     Gerd Hoffmann <kraxel@xxxxxxxxxx>
AuthorDate: Wed Feb 22 16:25:19 2017 +0000
Commit:     Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CommitDate: Wed Feb 22 16:26:27 2017 +0000

    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 ]
    
    Fixed compilation by removing extra parameter to blit_is_unsafe. -iwj
    
    Reported-by: Gerd Hoffmann <ghoffman@xxxxxxxxxx>
    Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
    Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
    (cherry picked from commit 83256b312acfddda34d96e9bba30e93387481091)
---
 hw/cirrus_vga.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/hw/cirrus_vga.c b/hw/cirrus_vga.c
index 364e22d..06b4a3b 100644
--- a/hw/cirrus_vga.c
+++ b/hw/cirrus_vga.c
@@ -899,6 +899,10 @@ static int cirrus_bitblt_cputovideo(CirrusVGAState * s)
 {
     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];
@@ -924,6 +928,10 @@ 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);
--
generated by git-patchbot for /home/xen/git/qemu-xen-traditional.git#stable-4.8

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