Re: [RFC PATCH v7 1/7] Restartable sequences system call

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Aug 10, 2016, at 3:16 PM, Andy Lutomirski luto@amacapital.net wrote:

> On Wed, Aug 10, 2016 at 12:04 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>> ----- On Aug 10, 2016, at 4:10 AM, Andy Lutomirski luto@amacapital.net wrote:
>>
>>> On Tue, Aug 9, 2016 at 1:06 PM, Mathieu Desnoyers
>>> <mathieu.desnoyers@efficios.com> wrote:
>>
>> <snip>
>>
>>>> Actually, we want copy_from_user() there. This executes upon
>>>> resume to user-space, so we can take a page fault is needed, so
>>>> no "inatomic" needed. I therefore suggest:
>>>
>>> Running the code below via exit_to_usermode_loop...
>>>
>>>>
>>>> static bool rseq_get_rseq_cs(struct task_struct *t,
>>>>                 void __user **start_ip,
>>>>                 void __user **post_commit_ip,
>>>>                 void __user **abort_ip)
>>>> {
>>>>         unsigned long ptr;
>>>>         struct rseq_cs __user *urseq_cs;
>>>>         struct rseq_cs rseq_cs;
>>>>
>>>>         if (__get_user(ptr, &t->rseq->rseq_cs))
>>>>                 return false;
>>>>         if (!ptr)
>>>>                 return true;
>>>> #ifdef CONFIG_COMPAT
>>>>         if (in_compat_syscall()) {
>>>>                 urseq_cs = compat_ptr((compat_uptr_t)ptr);
>>>>                 if (copy_from_user(&rseq_cs, urseq_cs, sizeof(*rseq_cs)))
>>>>                         return false;
>>>>                 *start_ip = compat_ptr((compat_uptr_t)rseq_cs.start_ip);
>>>>                 *post_commit_ip = compat_ptr((compat_uptr_t)rseq_cs.post_commit_ip);
>>>>                 *abort_ip = compat_ptr((compat_uptr_t)rseq_cs.abort_ip);
>>>>                 return true;
>>>>         }
>>>> #endif
>>>
>>> ...means that in_compat_syscall() is nonsense.  (It *works* there, but
>>> I can't imagine that it does anything that is actually sensible for
>>> this use.)
>>
>> Agreed that we are not per-se in a system call here. It works for
>> in_ia32_syscall(), but it may not work for in_x32_syscall().
>>
>> Then should we test for this ?
>>
>> if (!is_64bit_mm(current->mm))
>>
>> This is currently x86-specific. Is this how we are expected to test
>> the user-space pointer size in the current mm in arch-agnostic code ?
>> If so, we should implement is_64bit_mm() on all other architectures.
> 
> There is no universal concept of the user-space pointer size on x86
> because x86 code can change it via long jumps.
> 
> What are you actually trying to do?  I would guess that
> user_64bit_mode(regs) is the right thing here, because the rseq data
> structure is describing the currently executing code.

Yes, that's correct, we care about the pointer size of currently executing
code. On x86 user_64bit_mode(regs) would appear to be the right thing to do.

> 
>>
>>>
>>> Can't you just define the ABI so that no compat junk is needed?
>>> (Also, CRIU will thank you for doing that.)
>>
>> We are dealing with user-space pointers here, so AFAIU we need to
>> be aware of their size, which involves compat code. Am I missing
>> something ?
> 
> u64 is a perfectly valid, if odd, userspace pointer on all
> architecures that I know of, and it's certainly a valid userspace
> pointer on x86 32-bit userspace (the high bits will just all be zero).
> Can you just use u64?

My concern is about a 32-bit user-space putting garbage rather than zeroes
(on purpose) to fool the kernel on those upper 32 bits. Doing

  compat_ptr((compat_uptr_t)rseq_cs.start_ip)

effectively ends up clearing the upper 32 bits.

But since we only use those pointer values for comparisons, perhaps we
just don't care if a 32-bit userspace app try to shoot itself in
the foot by passing garbage upper 32 bits ?

> 
> If this would be a performance problem on ARM, then maybe that's a
> reason to use compat helpers.

We already use 64-bit values for the pointers, even on 32-bit. Normally
userspace just puts zeroes in the top bits. It's mostly a question of
clearing the top 32 bits or not when loading them in the kernel. If we
don't need to, then I can remove the compat code entirely, and we don't
care about user_64bit_mode() anymore, as you initially recommended.
Does it make sense ?

> 
>>
>>>
>>>
>>>>>> +SYSCALL_DEFINE2(rseq, struct rseq __user *, rseq, int, flags)
>>>>>> +{
>>>>>> +    if (unlikely(flags))
>>>>>> +            return -EINVAL;
>>>>>
>>>>> (add whitespace)
>>>>
>>>> fixed.
>>>>
>>>>>
>>>>>> +    if (!rseq) {
>>>>>> +            if (!current->rseq)
>>>>>> +                    return -ENOENT;
>>>>>> +            return 0;
>>>>>> +    }
>>>
>>> This looks entirely wrong.  Setting rseq to NULL fails if it's already
>>> NULL but silently does nothing if rseq is already set?  Surely it
>>> should always succeed and it should actually do something if rseq is
>>> set.
>>
>> From the proposed rseq(2) manpage:
>>
>> "A NULL rseq value can be used to check whether rseq is registered
>> for the current thread."
>>
>> The implementation does just that: it returns -1, errno=ENOENT if no
>> rseq is currently registered, or 0 if rseq is currently registered.
> 
> I think that's problematic.  Why can't you unregister an existing
> rseq?  If you can't, how is a thread supposed to clean up after
> itself?
> 

Unregistering an existing thread rseq would require that we keep reference
counting, in case multiple libs and/or the app are using rseq. I am
trying to keep things as simple as needed.

If I understand your concern, the problematic scenario would be at
thread exit (this is my current approximate understanding of glibc
handling of library TLS variable reclaim at thread exit):

thread exits in userspace:
- glibc frees its rseq TLS memory area (in case the TLS is in a library),
- thread preempted before really exiting,
- kernel reads/writes to freed TLS memory.
  - corruption may occur (e.g. memory re-allocated by another thread already)

Am I getting it right ?

Thanks,

Mathieu

> --Andy

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com