Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

[David Hildenbrand]

On 14.10.21 22:16, Suren Baghdasaryan wrote:
> On Tue, Oct 12, 2021 at 10:01 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>>
>> On Tue, Oct 12, 2021 at 12:44 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>
>>>> I'm still evaluating the proposal to use memfds but I'm not sure if
>>>> the issue that David Hildenbrand mentioned about additional memory
>>>> consumed in pagecache (which has to be addressed) is the only one we
>>>> will encounter with this approach. If anyone knows of any potential
>>>> issues with using memfds as named anonymous memory, I would really
>>>> appreciate your feedback before I go too far in that direction.
>>>
>>> [MAP_PRIVATE memfd only behave that way with 4k, not with huge pages, so
>>> I think it just has to be fixed. It doesn't make any sense to allocate a
>>> page for the pagecache ("populate the file") when accessing via a
>>> private mapping that's supposed to leave the file untouched]
>>>
>>> My gut feeling is if you really need a string as identifier, then try
>>> going with memfds. Yes, we might hit some road blocks to be sorted out,
>>> but it just logically makes sense to me: Files have names. These names
>>> exist before mapping and after mapping. They "name" the content.
>>
>> I'm investigating this direction. I don't have much background with
>> memfds, so I'll need to digest the code first.
> 
> I've done some investigation into the possibility of using memfds to
> name anonymous VMAs. Here are my findings:

Thanks for exploring the alternatives!

> 
> 1. Forking a process with anonymous vmas named using memfd is 5-15%
> slower than with prctl (depends on the number of VMAs in the process
> being forked). Profiling shows that i_mmap_lock_write() dominates
> dup_mmap(). Exit path is also slower by roughly 9% with
> free_pgtables() and fput() dominating exit_mmap(). Fork performance is
> important for Android because almost all processes are forked from
> zygote, therefore this limitation already makes this approach
> prohibitive.

Interesting, naturally I wonder if that can be optimized.

> 
> 2. mremap() usage to grow the mapping has an issue when used with memfds:
> 
> fd = memfd_create(name, MFD_ALLOW_SEALING);
> ftruncate(fd, size_bytes);
> ptr = mmap(NULL, size_bytes, prot, MAP_PRIVATE, fd, 0);
> close(fd);
> ptr = mremap(ptr, size_bytes, size_bytes * 2, MREMAP_MAYMOVE);
> touch_mem(ptr, size_bytes * 2);
> 
> This would generate a SIGBUS in touch_mem(). I believe it's because
> ftruncate() specified the size to be size_bytes and we are accessing
> more than that after remapping. prctl() does not have this limitation
> and we do have a usecase for growing a named VMA.

Can't you simply size the memfd much larger? I mean, it doesn't really
cost much, does it?

> 
> 3. Leaves an fd exposed, even briefly, which may lead to unexpected
> flaws (e.g. anything using mmap MAP_SHARED could allow exposures or
> overwrites). Even MAP_PRIVATE, if an attacker writes into the file
> after ftruncate() and before mmap(), can cause private memory to be
> initialized with unexpected data.

I don't quite follow. Can you elaborate what exactly the issue here is?
We use a temporary fd, yes, but how is that a problem?

Any attacker can just write any random memory memory in the address
space, so I don't see the issue.

> 
> 4. There is a usecase in the Android userspace where vma naming
> happens after memory was allocated. Bionic linker does in-memory
> relocations and then names some relocated sections.

Would renaming a memfd be an option or is that "too late" ?

> 
> In the light of these findings, could the current patchset be reconsidered?
> Thanks,
> Suren.
> 


-- 
Thanks,

David / dhildenb