Re: [PATCH 4/6] hugetlbfs: implement memfd sealing

[Mike Kravetz]

On 11/03/2017 10:41 AM, David Herrmann wrote:
> Hi
> 
> On Fri, Nov 3, 2017 at 6:12 PM, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>> On 11/03/2017 10:03 AM, David Herrmann wrote:
>>> Hi
>>>
>>> On Tue, Oct 31, 2017 at 7:40 PM, Marc-Andrà Lureau
>>> <marcandre.lureau@xxxxxxxxxx> wrote:
>>>> Implements memfd sealing, similar to shmem:
>>>> - WRITE: deny fallocate(PUNCH_HOLE). mmap() write is denied in
>>>>   memfd_add_seals(). write() doesn't exist for hugetlbfs.
>>>> - SHRINK: added similar check as shmem_setattr()
>>>> - GROW: added similar check as shmem_setattr() & shmem_fallocate()
>>>>
>>>> Except write() operation that doesn't exist with hugetlbfs, that
>>>> should make sealing as close as it can be to shmem support.
>>>
>>> SEAL, SHRINK, and GROW look fine to me.
>>>
>>> Regarding WRITE
>>
>> The commit message may not be clear.  However, hugetlbfs does not support
>> the write system call (or aio).  The only way to modify contents of a
>> hugetlbfs file is via mmap or hole punch/truncate.  So, we do not really
>> need to worry about those special (a)io cases for hugetlbfs.
> 
> This is not about the write(2) syscall. Please consider this scenario
> about shmem:
> 
> You create a memfd via memfd_create() and map it writable. You now
> call another kernel syscall that takes as input _any mapped page
> range_. You pass your mapped memfd-addresses to it. Those syscalls
> tend to use get_user_pages() to pin arbitrary user-mapped pages, as
> such this also affects shmem. In this case, those pages might stay
> mapped even if you munmap() your memfd!
> 
> One example of this is using AIO-read() on any other file that
> supports it, passing your mapped memfd as buffer to _read into_. The
> operations supported on the memfd are irrelevant here.
> The selftests contain a FUSE-based test for this, since FUSE allows
> user-space to GUP pages for an arbitrary amount of time.
> 
> The original fix for this is:
> 
>     commit 05f65b5c70909ef686f865f0a85406d74d75f70f
>     Author: David Herrmann <dh.herrmann@xxxxxxxxx>
>     Date:   Fri Aug 8 14:25:36 2014 -0700
> 
>         shm: wait for pins to be released when sealing
> 
> Please have a look at this. Your patches use shmem_add_seals() almost
> unchanged, and as such you call into shmem_wait_for_pins() on
> hugetlbfs. I would really like to see an explicit ACK that this works
> on hugetlbfs.

Thanks for the explanation.  I missed that in your first reply.  I'll
look into this for hugetlbfs.

-- 
Mike Kravetz

> 
> Thanks
> David
> 
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