Re: [RFC] arm64: Detecting tagged addresses

[Vincenzo Frascino]

+ Dan and smatch@xxxxxxxxxxxxxxx

Hi Andrew,

I am adding Dan to this thread since he is the smatch maintainer, and the
smatch@xxxxxxxxxxxxxxx list.

@Dan and @smatch@xxxxxxxxxxxxxxx: a reference to the beginning of this thread
can be found at [1].

[1] https://lkml.org/lkml/2019/6/19/376

On 6/27/19 2:18 PM, Andrew Murray wrote:
> On Wed, Jun 26, 2019 at 06:45:03PM +0100, Catalin Marinas wrote:
>> Hi Andrew,
>>
>> Cc'ing Luc (sparse maintainer) who's been involved in the past
>> discussions around static checking of user pointers:
>>
>> https://lore.kernel.org/linux-arm-kernel/20180905190316.a34yycthgbamx2t3@xxxxxxxxxx/
>>
>> So I think the difference here from the previous approach is that we
>> explicitly mark functions that cannot take tagged addresses (like
>> find_vma()) and identify the callers.
> 
> Indeed.
> 
> 
>>
>> More comments below:
>>
>> On Wed, Jun 19, 2019 at 01:16:20PM +0100, Andrew Murray wrote:
>>> The proposed introduction of a relaxed ARM64 ABI [1] will allow tagged memory
>>> addresses to be passed through the user-kernel syscall ABI boundary. Tagged
>>> memory addresses are those which contain a non-zero top byte (the hardware
>>> has always ignored this top byte due to TCR_EL1.TBI0) and may be useful
>>> for features such as HWASan.
>>>
>>> To permit this relaxation a proposed patchset [2] strips the top byte (tag)
>>> from user provided memory addresses prior to use in kernel functions which
>>> require untagged addresses (for example comparasion/arithmetic of addresses).
>>> The author of this patchset relied on a variety of techniques [2] (such as
>>> grep, BUG_ON, sparse etc) to identify as many instances of possible where
>>> tags need to be stipped.
>>>
>>> To support this effort and to catch future regressions (e.g. in new syscalls
>>> or ioctls), I've devised an additional approach for detecting the use of
>>> tagged addresses in functions that do not want them. This approach makes
>>> use of Smatch [3] and is outlined in this RFC. Due to the ability of Smatch
>>> to do flow analysis I believe we can annotate the kernel in fewer places
>>> than a similar approach in sparse.
>>>
>>> I'm keen for feedback on the likely usefulness of this approach.
>>>
>>> We first add some new annotations that are exclusively consumed by Smatch:
>>>
>>> --- a/include/linux/compiler_types.h
>>> +++ b/include/linux/compiler_types.h
>>> @@ -19,6 +19,7 @@
>>>  # define __cond_lock(x,c)      ((c) ? ({ __acquire(x); 1; }) : 0)
>>>  # define __percpu      __attribute__((noderef, address_space(3)))
>>>  # define __rcu         __attribute__((noderef, address_space(4)))
>>> +# define __untagged    __attribute__((address_space(5)))
>>>  # define __private     __attribute__((noderef))
>>>  extern void __chk_user_ptr(const volatile void __user *);
>>>  extern void __chk_io_ptr(const volatile void __iomem *);
>> [...]
>>> --- a/mm/mmap.c
>>> +++ b/mm/mmap.c
>>> @@ -2224,7 +2224,7 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>>>  EXPORT_SYMBOL(get_unmapped_area);
>>>  
>>>  /* Look up the first VMA which satisfies  addr < vm_end,  NULL if none. */
>>> -struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>>> +struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long __untagged addr)
>>>  {
>>>         struct rb_node *rb_node;
>>>         struct vm_area_struct *vma;
>> [...]
>>> This can be further improved - the problem here is that for a given function,
>>> e.g. find_vma we look for callers where *any* of the parameters
>>> passed to find_vma are tagged addresses from userspace - i.e. not *just*
>>> the annotated parameter. This is also true for find_vma's callers' callers'.
>>> This results in the call tree having false positives.
>>>
>>> It *is* possible to track parameters (e.g. find_vma arg 1 comes from arg 3 of
>>> do_pages_stat_array etc), but this is limited as if functions modify the
>>> data then the tracking is stopped (however this can be fixed).
>> [...]
>>> An example of a false positve is do_mlock. We untag the address and pass that
>>> to apply_vma_lock_flags - however we also pass a length - because the length
>>> came from userspace and could have the top bits set - it's flagged. However
>>> with improved parameter tracking we can remove this false positive and similar.
>>
>> Could we track only the conversions from __user * that eventually end up
>> as __untagged? (I'm not familiar with smatch, so not sure what it can
>> do).
> 
> I assume you mean 'that eventually end up as an argument annotated __untagged'?
> 
> The warnings smatch currently produce relate to only the conversions you
> mention - however further work is needed in smatch to improve the scripts that
> retrospectively provide call traces (without false positives).
> 
> 
>> We could assume that an unsigned long argument to a syscall is
>> default __untagged, unless explicitly marked as __tagged. For example,
>> sys_munmap() is allowed to take a tagged address.
> 
> I'll give this some further thought.
> 
> 
>>
>>> Prior to smatch I attempted a similar approach with sparse - however it seemed
>>> necessary to propogate the __untagged annotation in every function up the call tree,
>>> and resulted in adding the __untagged annotation to functions that would never
>>> get near user provided data. This leads to a littering of __untagged all over the
>>> kernel which doesn't seem appealing.
>>
>> Indeed. We attempted this last year (see the above thread).
>>
>>> Smatch is more capable, however it almost
>>> certainly won't pick up 100% of issues due to the difficulity of making flow
>>> analysis understand everything a compiler can.
>>>
>>> Is it likely to be acceptable to use the __untagged annotation in user-path
>>> functions that require untagged addresses across the kernel?
>>
>> If it helps with identifying missing untagged_addr() calls, I would say
>> yes (as long as we keep them to a minimum).
> 
> Thanks for the feedback.
> 
> Andrew Murray
> 
>>
>>> [1] https://lkml.org/lkml/2019/6/13/534
>>> [2] https://patchwork.kernel.org/cover/10989517/
>>> [3] http://smatch.sourceforge.net/
>>
>> -- 
>> Catalin

-- 
Regards,
Vincenzo