Messages in this thread |  | | Subject | Re: RFC v2: post-init-read-only protection for data allocated dynamically | From | Igor Stoppa <> | Date | Thu, 4 May 2017 11:17:25 +0300 |
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Hi, I suspect this was accidentally a Reply-To instead of a Reply-All, so I'm putting back the CCs that were dropped.
On 03/05/17 21:41, Dave Hansen wrote: > On 05/03/2017 05:06 AM, Igor Stoppa wrote: >> My starting point are the policy DB of SE Linux and the LSM Hooks, but >> eventually I would like to extend the protection also to other >> subsystems, in a way that can be merged into mainline. > > Have you given any thought to just having a set of specialized slabs?
No, the idea of the RFC was to get this sort of comments about options I might have missed :-)
> Today, for instance, we have a separate set of kmalloc() slabs for DMA: > dma-kmalloc-{4096,2048,...}. It should be quite possible to have > another set for your post-init-read-only protected data.
I will definitely investigate it and report back, thanks. But In the meanwhile I'd appreciate further clarifications. Please see below ...
> This doesn't take care of vmalloc(), but I have the feeling that > implementing this for vmalloc() isn't going to be horribly difficult.
ok
>> * The mechanism used for locking down the memory region is to program >> the MMU to trap writes to said region. It is fairly efficient and >> HW-backed, so it doesn't introduce any major overhead, > > I'd take a bit of an issue with this statement. It *will* fracture > large pages unless you manage to pack all of these allocations entirely > within a large page. This is problematic because we use the largest > size available, and that's 1GB on x86.
I am not sure I fully understand this part. I am probably missing some point about the way kmalloc works.
I get the problem you describe, but I do not understand why it should happen. Going back for a moment to my original idea of the zone, as a physical address range, why wouldn't it be possible to define it as one large page?
Btw, I do not expect to have much memory occupation, in terms of sheer size, although there might be many small "variables" scattered across the code. That's where I hope using kmalloc, instead of a custom made allocator can make a difference, in terms of optimal occupation.
> IOW, if you scatter these things throughout the address space, you may > end up fracturing/demoting enough large pages to cause major overhead > refilling the TLB.
But why would I? Or, better, what would cause it, unless I take special care?
Or, let me put it differently: my goal is to not fracture more pages than needed. It will probably require some profiling to figure out what is the ballpark of the memory footprint.
I might have overlooked some aspect of this, but the overall goal is to have a memory range (I won't call it zone, to avoid referring to a specific implementation) which is as tightly packed as possible, stuffed with all the data that is expected to become read-only.
> Note that this only applies for kmalloc() allocations, *not* vmalloc() > since kmalloc() uses the kernel linear map and vmalloc() uses it own, > separate mappings.
Yes.
--- thanks, igor
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