[kernel-hardening] [RFC] memory allocations in genalloc

[Laura Abbott]

On 08/17/2017 09:26 AM, Igor Stoppa wrote:
> Foreword:
> If I should direct this message to someone else, please let me know.
> I couldn't get a clear idea, by looking at both MAINTAINERS and git blame.
> 
> ****
> 
> Hi,
> 
> I'm currently trying to convert the SE Linux policy db into using a
> protectable memory allocator (pmalloc) that I have developed.
> 
> Such allocator is based on genalloc: I had come up with an
> implementation that was pretty similar to what genalloc already does, so
> it was pointed out to me that I could have a look at it.
> 
> And, indeed, it seemed a perfect choice.
> 
> But ... when freeing memory, genalloc wants that the caller also states
> how large each specific memory allocation is.
> 
> This, per se, is not an issue, although genalloc doesn't seen to check
> if the memory being freed is really matching a previous allocation request.
> 
> However, this design doesn't sit well with the use case I have in mind.
> 
> In particular, when the SE Linux policy db is populated, the creation of
> one or more specific entry of the db might fail.
> In this case, the memory previously allocated for said entry, is
> released with kfree, which doesn't need to know the size of the chunk
> being freed.
> 
> I would like to add similar capability to genalloc.
> 
> genalloc already uses bitmaps, to track what words are allocated (1) and
> which are free (0)
> 
> What I would like to do is to add another bitmap, which would track the
> beginning of each individual allocation (1 on the first allocation unit
> of each allocation, 0 otherwise).
> 
> Such enhancement would enable also the detection of calls to free with
> incorrect / misaligned addresses - right now it is possible to
> successfully free a memory area that overlaps the interface of two
> adjacent allocations, without fully covering either of them.
> 
> Would this change be acceptable?
> Is there any better way to achieve what I want?
> 

In general, I don't see anything wrong with wanting to let gen_pool_free
not take a size. It's hard to say anything more without a patch to review.
My biggest concern would be keeping existing behavior and managing two
bitmaps locklessly.


> 
> ---
> 
> I have also a question wrt the use of spinlocks in genalloc.
> Why a spinlock?
> 
> Freeing a chunk of memory previously allocated with vmalloc requires
> invoking vfree_atomic, instead of vfree, because the list of chunks is
> walked with the spinlock held, and vfree can sleep.
> 
> Why not using a mutex?
> 

>From the git history, gen_pool used to use a reader/writer lock and
was switched to be lockless so it could be used in NMI contexts
7f184275aa30 ("lib, Make gen_pool memory allocator lockless").
This looks to be an intentional choice, presumably so regions can be
added in atomic contexts. Again, if you have a specific patch or
proposal this would be easier to review.

Thanks,
Laura


> 
> --
> TIA, igor
> 

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