[Qemu-devel] Re: [PATCH to consider for 0.12] vmware_vga: Don't crash on too-big DEFINE_CURSOR command

[Roland Dreier <rdreier@cisco.com>]

 > Thanks for the patch.  I'm planning on giving Dave Airlie's series a
 > try for 0.12.0.  I'm pretty comfortable with those patches (since a
 > few of them are mine :-)).  I also don't think vmware-vga is going to
 > be reliable without them so I don't think pulling in the one fix is
 > good enough.
 > 
 > His last patch has the same fix without the printf().  The printf is
 > probably something to avoid since a malicious guest could create a
 > storm of them.  Since libvirt logs stderr by default, the result could
 > be pretty nasty.

Fair enough... I just saw Dave's patches go by, and I guess we
independently fixed the cursor size thing at right around the same time.
How about the following, without the fprintf but with paranoid checks
(since a malicious guest could send a bad DEFINE_CURSOR and do who knows
what with the buffer overrun, which is even worse than spamming logs ;)

====

QEMU crashes with vmware_vga when running a Linux guest with the latest
X.org vmware video driver if QEMU is using SDL for video output.  In
this case, vmware_vga advertises cursor acceleration to the guest, and
the crash comes when the guest does a DEFINE_CURSOR command with a 64x64
32bpp cursor.  This request overruns the image[] array in struct
vmsvga_cursor_definition_s and QEMU ends up segfaulting because of
memory corruption caused by writing past the end of the array.

Fix this by enlarging the image[] array to be able to hold 4096 32-bit
pixels so we don't fail for the case of 64*64*32bpp, and also add error
checking to avoid a crash if an even bigger request is sent by a guest.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
---
 hw/vmware_vga.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/hw/vmware_vga.c b/hw/vmware_vga.c
index f3e3749..75d90f2 100644
--- a/hw/vmware_vga.c
+++ b/hw/vmware_vga.c
@@ -462,7 +462,7 @@ struct vmsvga_cursor_definition_s {
     int hot_x;
     int hot_y;
     uint32_t mask[1024];
-    uint32_t image[1024];
+    uint32_t image[4096];	/* allow for 64x64 32bpp cursor */
 };
 
 #define SVGA_BITMAP_SIZE(w, h)		((((w) + 31) >> 5) * (h))
@@ -557,6 +557,13 @@ static void vmsvga_fifo_run(struct vmsvga_state_s *s)
             cursor.height = y = vmsvga_fifo_read(s);
             vmsvga_fifo_read(s);
             cursor.bpp = vmsvga_fifo_read(s);
+
+	    if (SVGA_BITMAP_SIZE(x, y) > sizeof cursor.mask ||
+		SVGA_PIXMAP_SIZE(x, y, cursor.bpp) > sizeof cursor.image) {
+		    args = SVGA_BITMAP_SIZE(x, y) + SVGA_PIXMAP_SIZE(x, y, cursor.bpp);
+		    goto badcmd;
+	    }
+
             for (args = 0; args < SVGA_BITMAP_SIZE(x, y); args ++)
                 cursor.mask[args] = vmsvga_fifo_read_raw(s);
             for (args = 0; args < SVGA_PIXMAP_SIZE(x, y, cursor.bpp); args ++)