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

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

On 05/18/2018 03:38 PM, Dan Williams wrote:
> On Fri, May 18, 2018 at 12:21 PM, Gustavo A. R. Silva
> <gustavo@embeddedor.com> wrote:
>>
>>
>> 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
> 
> I think this is fine in concept, we already do something similar in
> mpls_label_ok(). Perhaps call it validate_index_nospec() since
> validation is something that can fail, but sanitization in theory is
> something that can always succeed.
> 

OK. I got it.

> However, the problem is the data type of the index. I expect you would
> need to do this in a macro and use typeof() if you wanted this to be
> generally useful, and also watch out for multiple usage of a macro
> argument. Is it still worth it at that point?
> 

Yeah. I think it is worth it. I'll work on this during the weekend and 
send a proper patch for review.

Thanks for the feedback.
--
Gustavo