Re: [PATCH] kernel: sys: fix potential Spectre v1

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 05/18/2018 02:04 PM, Gustavo A. R. Silva wrote:
> 
> 
> On 05/15/2018 05:57 PM, Dan Williams wrote:
>> On Tue, May 15, 2018 at 3:29 PM, Thomas Gleixner <tglx@linutronix.de> 
>> wrote:
>>> On Tue, 15 May 2018, Andrew Morton wrote:
>>>> On Mon, 14 May 2018 22:00:38 -0500 "Gustavo A. R. Silva" 
>>>> <gustavo@embeddedor.com> wrote:
>>>>
>>>>> resource can be controlled by user-space, hence leading to a
>>>>> potential exploitation of the Spectre variant 1 vulnerability.
>>>>>
>>>>> This issue was detected with the help of Smatch:
>>>>>
>>>>> kernel/sys.c:1474 __do_compat_sys_old_getrlimit() warn: potential
>>>>> spectre issue 'get_current()->signal->rlim' (local cap)
>>>>> kernel/sys.c:1455 __do_sys_old_getrlimit() warn: potential spectre 
>>>>> issue
>>>>> 'get_current()->signal->rlim' (local cap)
>>>>>
>>>>> Fix this by sanitizing *resource* before using it to index
>>>>> current->signal->rlim
>>>>>
>>>>> Notice that given that speculation windows are large, the policy is
>>>>> to kill the speculation on the first load and not worry if it can be
>>>>> completed with a dependent load/store [1].
>>>>
>>>> hm.  Not my area, but I'm always willing to learn ;)
>>>>
>>>>> --- a/kernel/sys.c
>>>>> +++ b/kernel/sys.c
>>>>> @@ -69,6 +69,9 @@
>>>>>   #include <asm/io.h>
>>>>>   #include <asm/unistd.h>
>>>>>
>>>>> +/* Hardening for Spectre-v1 */
>>>>> +#include <linux/nospec.h>
>>>>> +
>>>>>   #include "uid16.h"
>>>>>
>>>>>   #ifndef SET_UNALIGN_CTL
>>>>> @@ -1451,6 +1454,7 @@ SYSCALL_DEFINE2(old_getrlimit, unsigned int, 
>>>>> resource,
>>>>>      if (resource >= RLIM_NLIMITS)
>>>>>              return -EINVAL;
>>>>>
>>>>> +   resource = array_index_nospec(resource, RLIM_NLIMITS);
>>>>>      task_lock(current->group_leader);
>>>>>      x = current->signal->rlim[resource];
>>>>
>>>> Can the speculation proceed past the task_lock()?  Or is the policy to
>>>> ignore such happy happenstances even if they are available?
>>>
>>> Locks are not in the way of speculation. Speculation has almost no 
>>> limits
>>> except serializing instructions. At least they respect the magic AND
>>> limitation in array_index_nospec().
>>
>> I'd say it another way, because they don't respect the magic AND, we
>> just make the result in the speculation path safe. So, it's controlled
>> speculation.
>>
> 
> Dan,
> 
> What do you think about adding the following function to the nospec API:
> 
> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
> index e791ebc..81e9a77 100644
> --- a/include/linux/nospec.h
> +++ b/include/linux/nospec.h
> @@ -55,4 +55,17 @@ static inline unsigned long 
> array_index_mask_nospec(unsigned long index,
>                                                                          \
>          (typeof(_i)) (_i & _mask);                                      \
>   })
> +
> +
> +#ifndef sanitize_index_nospec
> +inline bool sanitize_index_nospec(unsigned long index,
> +                                 unsigned long size)
> +{
> +       if (index >= size)
> +               return false;
> +       index = array_index_nospec(index, size);
> +
> +       return true;
> +}
> +#endif
>   #endif /* _LINUX_NOSPEC_H */
> 

Oops, it seems I sent the wrong patch. The function would look like this:

#ifndef sanitize_index_nospec
inline bool sanitize_index_nospec(unsigned long *index,
                                   unsigned long size)
{
         if (*index >= size)
                 return false;
         *index = array_index_nospec(*index, size);

         return true;
}
#endif

Thanks
--
Gustavo