[Qemu-devel] [Bug 575887] Re: VNC heap corruption at 1400x1050 (with % 16 != 0)

[Anthony Liguori]

Marking this invalid against qemu as it doesn't support that non-
standard VESA resolution.

** Changed in: qemu
       Status: New => Confirmed

** Changed in: qemu
   Importance: Undecided => High

** Changed in: qemu
       Status: Confirmed => Invalid

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VNC heap corruption at 1400x1050 (with % 16 != 0)
https://bugs.launchpad.net/bugs/575887
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Status in QEMU: Invalid

Bug description:
vnc_refresh_server_surface assumes that the display width
is a multiple of 16.  If it's not, then it accesses beyond the end of the row
by a few bytes.  On all but the last row, this is mostly harmless (it can
result in unnecessarily marking the end of the row dirty), but on the last row,
it copies over heap metadata.  This triggers a crash when changing resolutions 
or disconnecting and reconnecting a client.

I can trigger this reliably with a Windows 7 guest at 1400x1050 with -vga std.

The attached patch (rather ugly, with debugging code for good measure) 
partially fixes the issue.  There's still a black stripe on the right side of 
the screen, presumably because there are other bugs in vnc.c (or I messed up 
the patch).

I'm marking this as a security vulnerability because it allows the guest to 
overwrite host memory.

The same issue is tracked in Red Hat's bugzilla here:
https://bugzilla.redhat.com/show_bug.cgi?id=583850