On Wed, Jan 25, 2017 at 11:35:44AM +0100, Laszlo Ersek wrote: > On 01/25/17 10:50, Gerd Hoffmann wrote: > > On Mi, 2017-01-25 at 09:30 +0100, Wolfgang Bumiller wrote: > >> On Wed, Jan 25, 2017 at 08:07:05AM +0100, Gerd Hoffmann wrote: > >>> From: Li Qiang <liqiang...@360.cn> > >>> > >>> When doing bitblt copy in backward mode, we should minus the > >>> blt width first just like the adding in the forward mode. This > >>> can avoid the oob access of the front of vga's vram. > >>> > >>> Signed-off-by: Li Qiang <liqiang...@360.cn> > >>> Message-id: 5887254f.863a240a.2c122.5...@mx.google.com > >>> > >>> { kraxel: with backward blits (negative pitch) addr is the topmost > >>> address, so check it as-is against vram size ] > >>> > >>> Cc: qemu-sta...@nongnu.org > >>> Cc: P J P <ppan...@redhat.com> > >>> Cc: Laszlo Ersek <ler...@redhat.com> > >>> Cc: Paolo Bonzini <pbonz...@redhat.com> > >>> Cc: Wolfgang Bumiller <w.bumil...@proxmox.com> > >>> Fixes: d3532a0db02296e687711b8cdc7791924efccea0 (CVE-2014-8106) > >>> Signed-off-by: Gerd Hoffmann <kra...@redhat.com> > >>> --- > >>> hw/display/cirrus_vga.c | 7 +++---- > >>> 1 file changed, 3 insertions(+), 4 deletions(-) > >>> > >>> diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c > >>> index 379910d..b8c29a6 100644 > >>> --- a/hw/display/cirrus_vga.c > >>> +++ b/hw/display/cirrus_vga.c > >>> @@ -277,10 +277,9 @@ static bool blit_region_is_unsafe(struct > >>> CirrusVGAState *s, > >>> } > >>> if (pitch < 0) { > >>> int64_t min = addr > >>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; > >>> - int32_t max = addr > >>> - + s->cirrus_blt_width; > >>> - if (min < 0 || max > s->vga.vram_size) { > >>> + + ((int64_t)s->cirrus_blt_height - 1) * pitch > >>> + - s->cirrus_blt_width; > >>> + if (min < 0 || addr > s->vga.vram_size) { > >> > >> Call me paranoid, but shouldn't this be '>='? Missed this yesterday > >> apparently, correct me if I'm wrong: > >> If VRAM goes from 0..7 it has a size of 8, and this would accept > >> address 8 as it's not > size. > > > > I think you are right. The bkwd ops first execute the op, then > > decrement, so addr is inclusive and the check is off by one. > > That's right IMO; however, in that case we also have to posit that "min" > is exclusive. Assume that we have 16 pixels in the VGA memory (4x4), and > that we are massaging the bottom right quadrant: > > 0 1 2 3 > 4 5 6 7 > 8 9 10 11 > 12 13 14 15 > > addr = 15 > height = 2 > width = 2 > pitch = -4 > > Then > > min = addr + (height - 1) * pitch - width > = 15 + ( 2 - 1) * (-4) - 2 > = 9 > > Which is the address right before the top left pixel; that is, it marks > the first pixel *not* accessed. > > If that value was (-1), then the operation would still be valid. > > So we should accept (min == -1) -- this is dictated by plain symmetry. > If "max" -- here, "addr" -- is inclusive, then "min" becomes exclusive.
You're right. You'd think it wouldn't take so many different people to notice these things :(. It was right there, I should have noticed it.