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SubjectRe: [PATCH 0/3] linux-next: mm: hardening: Track genalloc allocations
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oops, sorry, I forgot the references :-(

On 29/04/18 20:39, Igor Stoppa wrote:
>
>
> On 29/04/18 07:09, Matthew Wilcox wrote:
>> On Sun, Apr 29, 2018 at 06:45:39AM +0400, Igor Stoppa wrote:
>>> This patchset was created as part of an older version of pmalloc,
>>> however
>>> it has value per-se, as it hardens the memory management for the generic
>>> allocator genalloc.
>>>
>>> Genalloc does not currently track the size of the allocations it
>>> hands out.
>>>
>>> Either by mistake, or due to an attack, it is possible that more memory
>>> than what was initially allocated is freed, leaving behind dangling
>>> pointers, ready for an use-after-free attack.
>>
>> This is a good point.  It is worth hardening genalloc.
>> But I still don't like the cost of the bitmap.  I've been
>> reading about allocators and I found Bonwick's paper from 2001:
>> https://www.usenix.org/legacy/event/usenix01/full_papers/bonwick/bonwick.pdf
>>
>> Section 4 describes the vmem allocator which would seem to have superior
>> performance and lower memory overhead than the current genalloc
>> allocator,
>> never mind the hardened allocator.
>>
>> Maybe there's been an advnace in resource allocator technology since
>> then that someone more familiar with CS research can point out.
>
> A quick search on google shows that there have been tons of improvements.
>
> I found various implementation of vmem, not all with GPLv2 compatible
> license.
>
> The most interesting one seems to be a libvmem from Intel[1], made to
> use jemalloc[2], for persistent memory.
>
> jemalloc is, apaprently, the coolest kid on the block, when it comes to
> modern memory management.
>
> But this is clearly a very large lump of work.
>
> First of all, it should be assessed if jemalloc is really what the
> kernel could benefit from (my guess is yes, but it's just a guess), then
> if the license is compatible or if it can be relicensed for use in the
> kernel.
>
> And, last, but not least, how to integrate the ongoing work in a way
> that doesn't require lots of effort to upgrade to new releases.
>
> Even if it looks very interesting, I simply do not have time to do this,
> not for the next 5-6 months, for sure.
>
> What I *can* offer to do, is the cleanup of the users of genalloc, by
> working with the various maintainers to remove the "size" parameter in
> the calls to gen_pool_free(), iff the patch I submitted can be merged.
>
> This is work that has to be done anyway and does not preclude, later on,
> to phase out genalloc in favor of jemalloc or whatever is deemed to be
> the most effective solution.
>
> There are 2 goals here:
> 1) plug potential security holes in the use of genalloc
> 2) see if some new allocator can improve the performance (and it might
> well be that the improvement can be extended also to other allocators
> used in kernel)
>
> We seem to agree that 1) is a real need.
> Regarding 2), I think it should have a separate life.
>
> going back to 1), your objections so far, as far as I can tell are:
>
> a) it will use more memory for the bitmap
> b) it will be slower, because the bitmap is doubled
> c) the "ends" or "starts" bitmaps should be separate
>
> I think I have already answered them, but I'll recap my replies:
>
> a) yes, it will double it, but if it was ok to "waste" some memory when
> I was asked to rewrite the pmalloc allocator to not use genalloc, in
> favor of speed, I think the same criteria applies here: on average it
> will probably take at most one more page per pool. It doesn't seem a
> huge loss.
>
> b) the bitmap size is doubled, that much is true, however interleaving
> the "busy" and "start" bitmaps will ensure locality of the meta data and
> between the cache prefetch algorithm and the hints give to the compiler,
> it shouldn't make a huge difference, compared to the pre-patch case.
> Oh, and the size of a bitmap seems to be overall negligible, from what
> users I saw.
>
> c) "busy" and "start" are interleaved, to avoid having to do explicit
> locking, instead of relying on the intrinsic atomicity of accessing
> bitfields coming from the same word, as it is now.
>
> And I'm anyway proposing to merge this into linux-next, so that there
> are more eyeballs looking for problems. I'm not proposing to merge it
> straight in the next kernel release.
> Removing the size parameter from the various gen_pool_free() will impact
> not only the direct callers, but also their callers and so on, which
> means that it will take some time to purge all the layers of calls from
> "size".
>
> During this time, it's likely that issues will surface, if there is any
> lurking.
>
> And the removal of the parameter will require getting ACK from each
> user, so it should be enough to ensure that everyone is happy about the
> overall performance.
>
> But I would start addressing the security issue, since I think the cost
> of doubling the bitmap will not be noticeable.
>
> I'd like to hear if you disagree with my reasoning.
>
> And did I overlook some other objection?
>

[1] https://github.com/pmem/pmdk/tree/master/src/libvmem
[2] http://jemalloc.net/

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