[kernel-hardening] Re: [PATCH v3 0/7] arm64: Privileged Access Never using TTBR0_EL1 switching

[Kees Cook]

On Thu, Oct 27, 2016 at 7:54 AM, Catalin Marinas
<catalin.marinas at arm.com> wrote:
> On Fri, Sep 30, 2016 at 11:42:02AM -0700, Kees Cook wrote:
>> On Thu, Sep 29, 2016 at 3:44 PM, Sami Tolvanen <samitolvanen at google.com> wrote:
>> > On Thu, Sep 15, 2016 at 05:20:45PM +0100, Mark Rutland wrote:
>> >> Likewise, how do we handle __flush_cache_user_range and
>> >> flush_icache_range? Some callers (e.g. __do_compat_cache_op) pass in
>> >> __user addresses.
>> >
>> > Also EXEC_USERSPACE in lkdtm passes a user space address to flush_icache_range
>> > and causes the process to hang when I tested these patches on HiKey.
>> >
>> > Adding uaccess_{enable,disable}_not_uao to __flush_cache_user_range appears to
>> > fix the problem.
>>
>> I had a thought just now on this: is lkdtm maybe doing the wrong thing
>> here? i.e. should lkdtm be the one do to the uaccess_en/disable
>> instead of flush_icache_range() itself, since it's the one abusing the
>> API?
>
> (preparing the v4 series)
>
> I think lkdtm is using the API incorrectly here. The documentation for
> flush_icache_range() (Documentation/cachetlb.txt) states that it is to
> be used on kernel addresses. Even with uaccess_enable/disable in lkdtm,
> faults on user space can still happen and the flush_icache_range()
> function must be able to handle them. It happens to work on arm64
> because of the fall through __flush_cache_user_range() but that's not
> guaranteed on other architectures.
>
> A potential solution is to use access_process_vm() and let the arch code
> handle the cache maintenance automatically:

Ah, perfect! I'll give this a spin, thanks!

-Kees

> ---------------------8<--------------------------------
> From fcbb7c9c30daf9bfc2a215ec10dba79c109ab835 Mon Sep 17 00:00:00 2001
> From: Catalin Marinas <catalin.marinas at arm.com>
> Date: Thu, 27 Oct 2016 15:47:20 +0100
> Subject: [PATCH] lkdtm: Do not use flush_icache_range() on user addresses
>
> The flush_icache_range() API is meant to be used on kernel addresses
> only as it may not have the infrastructure (exception entries) to handle
> user memory faults.
>
> The lkdtm execute_user_location() function tests the kernel execution of
> user space addresses by mmap'ing an anonymous page, copying some code
> together with cache maintenance and attempting to run it. However, the
> cache maintenance step may fail because of the incorrect API usage
> described above. The patch changes lkdtm to use access_process_vm() for
> copying the code into user space which would take care of the necessary
> cache maintenance.
>
> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> ---
>  drivers/misc/lkdtm_perms.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/misc/lkdtm_perms.c b/drivers/misc/lkdtm_perms.c
> index 45f1c0f96612..c7635a79341f 100644
> --- a/drivers/misc/lkdtm_perms.c
> +++ b/drivers/misc/lkdtm_perms.c
> @@ -60,15 +60,18 @@ static noinline void execute_location(void *dst, bool write)
>
>  static void execute_user_location(void *dst)
>  {
> +       int copied;
> +
>         /* Intentionally crossing kernel/user memory boundary. */
>         void (*func)(void) = dst;
>
>         pr_info("attempting ok execution at %p\n", do_nothing);
>         do_nothing();
>
> -       if (copy_to_user((void __user *)dst, do_nothing, EXEC_SIZE))
> +       copied = access_process_vm(current, (unsigned long)dst, do_nothing,
> +                                  EXEC_SIZE, FOLL_WRITE);
> +       if (copied < EXEC_SIZE)
>                 return;
> -       flush_icache_range((unsigned long)dst, (unsigned long)dst + EXEC_SIZE);
>         pr_info("attempting bad execution at %p\n", func);
>         func();
>  }



-- 
Kees Cook
Nexus Security