Re: [Qemu-devel] [PATCH 8/8] hw/i386: fix unbounded stack for load_multiboot

[Peter Maydell]

On 8 March 2016 at 14:00, Peter Xu <address@hidden> wrote:
> Suggested-by: Paolo Bonzini <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Richard Henderson <address@hidden>
> CC: Eduardo Habkost <address@hidden>
> CC: "Michael S. Tsirkin" <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  hw/i386/multiboot.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/hw/i386/multiboot.c b/hw/i386/multiboot.c
> index 9e164e6..0eecb9a 100644
> --- a/hw/i386/multiboot.c
> +++ b/hw/i386/multiboot.c
> @@ -159,6 +159,12 @@ int load_multiboot(FWCfgState *fw_cfg,
>      uint8_t *mb_bootinfo_data;
>      uint32_t cmdline_len;
>
> +#define __KERN_FNAME_LEN (1024)
> +#define __KERN_CMDLINE_LEN (4096)

Where do these choices of length restriction come from?

> +
> +    assert(strlen(kernel_filename) + 1 >= __KERN_FNAME_LEN);
> +    assert(strlen(kernel_cmdline) + 1 >= __KERN_CMDLINE_LEN);
> +
>      /* Ok, let's see if it is a multiboot image.
>         The header is 12x32bit long, so the latest entry may be 8192 - 48. */
>      for (i = 0; i < (8192 - 48); i += 4) {
> @@ -324,7 +330,7 @@ int load_multiboot(FWCfgState *fw_cfg,
>      }
>
>      /* Commandline support */
> -    char kcmdline[strlen(kernel_filename) + strlen(kernel_cmdline) + 2];
> +    char kcmdline[__KERN_FNAME_LEN + __KERN_CMDLINE_LEN];
>      snprintf(kcmdline, sizeof(kcmdline), "%s %s",
>               kernel_filename, kernel_cmdline);

Why can't we just dynamically allocate the command line
(ie g_strdup_printf() it) ?

>      stl_p(bootinfo + MBI_CMDLINE, mb_add_cmdline(&mbs, kcmdline));
> @@ -370,4 +376,6 @@ int load_multiboot(FWCfgState *fw_cfg,
>      nb_option_roms++;
>
>      return 1; /* yes, we are multiboot */
> +#undef __KERN_FNAME_LEN
> +#undef __KERN_CMDLINE_LEN

thanks
-- PMM