From: Gerd Hoffmann Date: Tue, 21 Feb 2017 18:54:59 +0000 (-0800) Subject: cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=722ce03b32f37ef5af09105727b574339326d354;p=qemu-upstream-4.6-testing.git 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 Signed-off-by: Gerd Hoffmann --- diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c index e2ee46a0c..d643a0da4 100644 --- a/hw/display/cirrus_vga.c +++ b/hw/display/cirrus_vga.c @@ -863,6 +863,10 @@ 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]; @@ -888,6 +892,10 @@ 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);