Re: [Qemu-devel] [PATCH 1/5] multiboot: Reject kernels exceeding the address space

[Jack Schwartz <jack.schwartz@oracle.com>]

Hi Kevin.

My comments are inline...

On 2018-03-14 10:32, Kevin Wolf wrote:
> The code path with a manually set mh_load_addr in the Multiboot header
> checks that load_end_addr <= load_addr, but the path where load_end_addr
> is automatically detected if 0 is given in the header misses the
> corresponding check.
1) The code checks that load_end_addr >= load_addr (before letting it
through).

2) load_end_addr is used only when it is read in as non-zero, so no 
check is needed if zero.  (It gets debug-printed even when zero, but is 
used only to calculate mb_load_size and only when non-zero.)
> If the kernel binary size is larger than can fit in
> the address space after load_addr, we ended up with a kernel_size that
> is smaller than load_size, which means that we read the file into a too
> small buffer.
>
> Add a check to reject kernel files with such Multiboot headers.
Code itself looks fine.

Modulo above comments:
     Reviewed-by: Jack Schwartz <jack.schwartz@oracle.com>

     Thanks,
     Jack
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   hw/i386/multiboot.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/hw/i386/multiboot.c b/hw/i386/multiboot.c
> index b9064264d8..1e215bf8d3 100644
> --- a/hw/i386/multiboot.c
> +++ b/hw/i386/multiboot.c
> @@ -247,6 +247,10 @@ int load_multiboot(FWCfgState *fw_cfg,
>               }
>               mb_load_size = kernel_file_size - mb_kernel_text_offset;
>           }
> +        if (mb_load_size > UINT32_MAX - mh_load_addr) {
> +            error_report("kernel does not fit in address space");
> +            exit(1);
> +        }
>           if (mh_bss_end_addr) {
>               if (mh_bss_end_addr < (mh_load_addr + mb_load_size)) {
>                   error_report("invalid bss_end_addr address");