Re: [PATCH] hw/core/loader: Fix possible crash in rom_copy()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Thomas,

On 9/25/19 3:03 PM, Thomas Huth wrote:
> Both, "rom->addr" and "addr" are derived from the binary image
> that can be loaded with the "-kernel" paramer. The code in
> rom_copy() then calculates:
> 
>     d = dest + (rom->addr - addr);
> 
> and uses "d" as destination in a memcpy() some lines later. Now with
> bad kernel images, it is possible that rom->addr is smaller than addr,
> thus "rom->addr - addr" gets negative and the memcpy() then tries to
> copy contents from the image to a bad memory location. In the best case,
> this just crashes QEMU, in the worst case, this could maybe be used to
> inject code from the kernel image into the QEMU binary, so we better fix
> it with an additional sanity check here.
> 
> Cc: qemu-stable@nongnu.org
> Reported-by: Guangming Liu
> Buglink: https://bugs.launchpad.net/qemu/+bug/1844635

"This page does not exist, or you may not have permission to see it."

This seems security related. Shouldn't we open a CVE for this?
https://wiki.qemu.org/SecurityProcess#CVE_allocation

Let's say I have write access to a LAN TFTP server used by some PXE
bootloader where I can store my crafted nasty kernel, then I get this score:

https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H/E:P/RL:O/RC:C&version=3.1

CVSS Base Score:     9.6
CVSS Temporal Score: 8.6

Which seems quite high.

> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  hw/core/loader.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/core/loader.c b/hw/core/loader.c
> index 0d60219364..5099f27dc8 100644
> --- a/hw/core/loader.c
> +++ b/hw/core/loader.c
> @@ -1281,7 +1281,7 @@ int rom_copy(uint8_t *dest, hwaddr addr, size_t size)

$ git show 235f86ef014
Date:   Thu Nov 12 21:53:11 2009 +0100

This function is old and poorly documented.

>          if (rom->addr + rom->romsize < addr) {
>              continue;
>          }
> -        if (rom->addr > end) {
> +        if (rom->addr > end || rom->addr < addr) {

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

>              break;
>          }
>  
>