Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

Khalid Aziz
2019-05-24T14:26:06Z

On 5/24/19 4:11 AM, Catalin Marinas wrote:
> On Thu, May 23, 2019 at 03:49:05PM -0600, Khalid Aziz wrote:
>> On 5/23/19 2:11 PM, Catalin Marinas wrote:
>>> On Thu, May 23, 2019 at 11:51:40AM -0600, Khalid Aziz wrote:
>>>> On 5/21/19 6:04 PM, Kees Cook wrote:
>>>>> As an aside: I think Sparc ADI support in Linux actually side-stepped
>>>>> this[1] (i.e. chose "solution 1"): "All addresses passed to kernel must
>>>>> be non-ADI tagged addresses." (And sadly, "Kernel does not enable ADI
>>>>> for kernel code.") I think this was a mistake we should not repeat for
>>>>> arm64 (we do seem to be at least in agreement about this, I think).
>>>>>
>>>>> [1] https://lore.kernel.org/patchwork/patch/654481/
>>>>
>>>> That is a very early version of the sparc ADI patch. Support for tagged
>>>> addresses in syscalls was added in later versions and is in the patch
>>>> that is in the kernel.
>>>
>>> I tried to figure out but I'm not familiar with the sparc port. How did
>>> you solve the tagged address going into various syscall implementations
>>> in the kernel (e.g. sys_write)? Is the tag removed on kernel entry or it
>>> ends up deeper in the core code?
>>
>> Another spot I should point out in ADI patch - Tags are not stored in
>> VMAs and IOMMU does not support ADI tags on M7. ADI tags are stripped
>> before userspace addresses are passed to IOMMU in the following snippet
>> from the patch:
>>
>> diff --git a/arch/sparc/mm/gup.c b/arch/sparc/mm/gup.c
>> index 5335ba3c850e..357b6047653a 100644
>> --- a/arch/sparc/mm/gup.c
>> +++ b/arch/sparc/mm/gup.c
>> @@ -201,6 +202,24 @@ int __get_user_pages_fast(unsigned long start, int
>> nr_pages
>> , int write,
>> pgd_t *pgdp;
>> int nr = 0;
>>
>> +#ifdef CONFIG_SPARC64
>> + if (adi_capable()) {
>> + long addr = start;
>> +
>> + /* If userspace has passed a versioned address, kernel
>> + * will not find it in the VMAs since it does not store
>> + * the version tags in the list of VMAs. Storing version
>> + * tags in list of VMAs is impractical since they can be
>> + * changed any time from userspace without dropping into
>> + * kernel. Any address search in VMAs will be done with
>> + * non-versioned addresses. Ensure the ADI version bits
>> + * are dropped here by sign extending the last bit before
>> + * ADI bits. IOMMU does not implement version tags.
>> + */
>> + addr = (addr << (long)adi_nbits()) >> (long)adi_nbits();
>> + start = addr;
>> + }
>> +#endif
>> start &= PAGE_MASK;
>> addr = start;
>> len = (unsigned long) nr_pages << PAGE_SHIFT;
>
> Thanks Khalid. I missed that sparc does not enable HAVE_GENERIC_GUP, so
> you fix this case here. If we add the generic untagged_addr() macro in
> the generic code, I think sparc can start making use of it rather than
> open-coding the shifts.

Hi Catalin,

Yes, that will be good. Right now addresses are untagged in sparc code
in only two spots but that will expand as we expand use of tags.
Scalabale solution is definitely better.

>
> There are a few other other places where tags can leak and the core code
> would get confused (for example, madvise()). I presume your user space
> doesn't exercise them. On arm64 we plan to just allow the C library to
> tag any new memory allocation, so those core code paths would need to be
> covered.
>
> And similarly, devices, IOMMU, any DMA would ignore tags.
>

Right. You are doing lot more with tags than sparc code intended to do.
I had looked into implementing just malloc(), mmap() and possibly
shmat() in library that automatically tags pointers. Expanding tags to
any pointers in C library will require covering lot more paths in kernel.

--
Khalid