Re: [PATCH v5 04/13] mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT_MEMLOCK

[David Hildenbrand]

On 08.04.22 19:56, Sean Christopherson wrote:
> On Thu, Apr 07, 2022, Andy Lutomirski wrote:
>>
>> On Thu, Apr 7, 2022, at 9:05 AM, Sean Christopherson wrote:
>>> On Thu, Mar 10, 2022, Chao Peng wrote:
>>>> Since page migration / swapping is not supported yet, MFD_INACCESSIBLE
>>>> memory behave like longterm pinned pages and thus should be accounted to
>>>> mm->pinned_vm and be restricted by RLIMIT_MEMLOCK.
>>>>
>>>> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
>>>> ---
>>>>  mm/shmem.c | 25 ++++++++++++++++++++++++-
>>>>  1 file changed, 24 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>> index 7b43e274c9a2..ae46fb96494b 100644
>>>> --- a/mm/shmem.c
>>>> +++ b/mm/shmem.c
>>>> @@ -915,14 +915,17 @@ static void notify_fallocate(struct inode *inode, pgoff_t start, pgoff_t end)
>>>>  static void notify_invalidate_page(struct inode *inode, struct folio *folio,
>>>>  				   pgoff_t start, pgoff_t end)
>>>>  {
>>>> -#ifdef CONFIG_MEMFILE_NOTIFIER
>>>>  	struct shmem_inode_info *info = SHMEM_I(inode);
>>>>  
>>>> +#ifdef CONFIG_MEMFILE_NOTIFIER
>>>>  	start = max(start, folio->index);
>>>>  	end = min(end, folio->index + folio_nr_pages(folio));
>>>>  
>>>>  	memfile_notifier_invalidate(&info->memfile_notifiers, start, end);
>>>>  #endif
>>>> +
>>>> +	if (info->xflags & SHM_F_INACCESSIBLE)
>>>> +		atomic64_sub(end - start, &current->mm->pinned_vm);
>>>
>>> As Vishal's to-be-posted selftest discovered, this is broken as current->mm
>>> may be NULL.  Or it may be a completely different mm, e.g. AFAICT there's
>>> nothing that prevents a different process from punching hole in the shmem
>>> backing.
>>>
>>
>> How about just not charging the mm in the first place?  There’s precedent:
>> ramfs and hugetlbfs (at least sometimes — I’ve lost track of the current
>> status).
>>
>> In any case, for an administrator to try to assemble the various rlimits into
>> a coherent policy is, and always has been, quite messy. ISTM cgroup limits,
>> which can actually add across processes usefully, are much better.
>>
>> So, aside from the fact that these fds aren’t in a filesystem and are thus
>> available by default, I’m not convinced that this accounting is useful or
>> necessary.
>>
>> Maybe we could just have some switch require to enable creation of private
>> memory in the first place, and anyone who flips that switch without
>> configuring cgroups is subject to DoS.
> 
> I personally have no objection to that, and I'm 99% certain Google doesn't rely
> on RLIMIT_MEMLOCK.
> 

It's unnacceptable for distributions to have random unprivileged users
be able to allocate an unlimited amount of unmovable memory. And any
kind of these "switches" won't help a thing because the distribution
will have to enable them either way.

I raised in the past that accounting might be challenging, so it's no
surprise that something popped up now.

RLIMIT_MEMLOCK was the obvious candidate, but as we discovered int he
past already with secretmem, it's not 100% that good of a fit (unmovable
is worth than mlocked). But it gets the job done for now at least.

So I'm open for alternative to limit the amount of unmovable memory we
might allocate for user space, and then we could convert seretmem as well.

Random switches are not an option.

-- 
Thanks,

David / dhildenb