Re: [PATCH v2] io_uring: Adjust mapping wrt architecture aliasing requirements

[Helge Deller <deller@gmx.de>]

On 2/16/23 17:46, Jens Axboe wrote:
> On 2/16/23 9:33?AM, Helge Deller wrote:
>> On 2/16/23 17:11, Jens Axboe wrote:
>>> On 2/16/23 1:09?AM, Helge Deller wrote:
>>>> Some architectures have memory cache aliasing requirements (e.g. parisc)
>>>> if memory is shared between userspace and kernel. This patch fixes the
>>>> kernel to return an aliased address when asked by userspace via mmap().
>>>>
>>>> Signed-off-by: Helge Deller <deller@gmx.de>
>>>> ---
>>>> v2: Do not allow to map to a user-provided addresss. This forces
>>>> programs to write portable code, as usually on x86 mapping to any
>>>> address will succeed, while it will fail for most provided address if
>>>> used on stricter architectures.
>>>>
>>>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>>> index 862e05e6691d..01fe7437a071 100644
>>>> --- a/io_uring/io_uring.c
>>>> +++ b/io_uring/io_uring.c
>>>> @@ -72,6 +72,7 @@
>>>>    #include <linux/io_uring.h>
>>>>    #include <linux/audit.h>
>>>>    #include <linux/security.h>
>>>> +#include <asm/shmparam.h>
>>>>
>>>>    #define CREATE_TRACE_POINTS
>>>>    #include <trace/events/io_uring.h>
>>>> @@ -3059,6 +3060,54 @@ static __cold int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
>>>>        return remap_pfn_range(vma, vma->vm_start, pfn, sz, vma->vm_page_prot);
>>>>    }
>>>>
>>>> +static unsigned long io_uring_mmu_get_unmapped_area(struct file *filp,
>>>> +            unsigned long addr, unsigned long len,
>>>> +            unsigned long pgoff, unsigned long flags)
>>>> +{
>>>> +    const unsigned long mmap_end = arch_get_mmap_end(addr, len, flags);
>>>> +    struct vm_unmapped_area_info info;
>>>> +    void *ptr;
>>>> +
>>>> +    /*
>>>> +     * Do not allow to map to user-provided address to avoid breaking the
>>>> +     * aliasing rules. Userspace is not able to guess the offset address of
>>>> +     * kernel kmalloc()ed memory area.
>>>> +     */
>>>> +    if (addr)
>>>> +        return -EINVAL;
>>>
>>> Can we relax this so that if the address is correctly aligned, it will
>>> allow it?
>>
>> My previous patch had it relaxed, but after some more thoughts I removed
>> it in this v2-version again.
>>
>> The idea behind it is good, but I see a huge disadvantage in allowing
>> correctly aligned addresses: People develop their code usually on x86
>> which has no such alignment requirements, as it just needs to be PAGE_SIZE aligned.
>> So their code will always work fine on x86, but as soon as the same code
>> is built on other platforms it will break. As you know, on parisc it's pure luck
>> if the program chooses an address which is correctly aligned.
>> I'm one of the debian maintainers for parisc, and I've seen similiar
>> mmap-issues in other programs as well. Everytime I've found it to be wrong,
>> you have to explain to the developers what's wrong and sometimes it's
>> not easy to fix it.
>> So, if we can educate people from assuming their code to be correct, I think
>> we can save a lot of additional work afterwards.
>> That said, I think it's better to be strict now, unless someone comes
>> up with a really good reason why it needs to be less strict.
>
> I don't disagree with the reasoning at all, but the problem is that it
> may introduce breakage if someone IS doing the right thing. Is it
> guaranteed to be true? No, certainly not. But someone could very well be
> writing perfectly portable code and mapping a ring into a specific
> address, and this will now break.

We will find out if there are such users if we keep it strict now and
open it up if it's really necessary.
If you open it up now, you won't be able to turn it stricter later.

> AFAICT, this is actually the case with the syzbot case. In fact, with
> the patch applied, it'll obviously start crashing on all archs as the
> mmaps will now return -EINVAL rather than work.

Yes, but it's not a real user and just a (invalid) testcase.
For that I think it's OK to just disable it.

Helge