Re: [PATCH 1/2] x86, mpx: explicitly disable 32-bit MPX support on 64-bit kernels

Andy Lutomirski
2014-12-22T20:18:17Z

On Mon, Dec 22, 2014 at 12:08 PM, Dave Hansen <dave@sr71.net> wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> We had originally planned on submitting MPX support in one patch
> set. We eventually broke it up in to two pieces for easier
> review. One of the features that didn't make the first round
> was supporting 32-bit binaries on 64-bit kernels.
>
> Once we split the set up, we never added code to restrict 32-bit
> binaries from _using_ MPX on 64-bit kernels.
>
> The 32-bit bounds tables are a different format than the 64-bit
> ones. Without this patch, the kernel will try to read a 32-bit
> binary's tables as if they were the 64-bit version. They will
> likely be noticed as being invalid rather quickly and the app
> will get killed, but that's kinda mean.
>
> This patch adds an explicit check, and will make a 64-bit kernel
> essentially behave as if it has no MPX support when called from
> a 32-bit binary.
>
> I intend to remove this check once the mixed binary support is
> added. Those patches are posted if anyone is interested:
>
> https://lkml.org/lkml/2014/12/12/632
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> ---
>
> b/arch/x86/mm/mpx.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff -puN arch/x86/mm/mpx.c~x86-mpx-explicitly-disable-32-bit-on-64-bit-for-3_19 arch/x86/mm/mpx.c
> --- a/arch/x86/mm/mpx.c~x86-mpx-explicitly-disable-32-bit-on-64-bit-for-3_19 2014-12-22 12:06:18.308911687 -0800
> +++ b/arch/x86/mm/mpx.c 2014-12-22 12:06:18.311911822 -0800
> @@ -349,6 +349,12 @@ static __user void *task_get_bounds_dir(
> return MPX_INVALID_BOUNDS_DIR;
>
> /*
> + * 32-bit binaries on 64-bit kernels are currently
> + * unsupported.
> + */
> + if (IS_ENABLED(CONFIG_X86_64) && test_thread_flag(TIF_IA32))
> + return MPX_INVALID_BOUNDS_DIR;

Should this check mm->ia32_compat instead?

Otherwise,

Reviewed-by: Andy Lutomirski <luto@amacapital.net>

> + /*
> * The bounds directory pointer is stored in a register
> * only accessible if we first do an xsave.
> */
> _

--
Andy Lutomirski
AMA Capital Management, LLC