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Date: Mon, 19 Mar 2018 20:04:35 +0200
From: Igor Stoppa <igor.stoppa@...wei.com>
To: Matthew Wilcox <willy@...radead.org>
CC: <keescook@...omium.org>, <david@...morbit.com>, <rppt@...ux.vnet.ibm.com>,
<mhocko@...nel.org>, <labbott@...hat.com>,
<linux-security-module@...r.kernel.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH v19 0/8] mm: security: ro protection for dynamic data
On 14/03/18 19:33, Matthew Wilcox wrote:
> I think an implementation of
> pmalloc which used a page_frag-style allocator would be larger than
> 100 lines, but I don't think it would have to be significantly larger
> than that.
I have some doubt about what is the best way to implement it using
vmalloced memory.
1. Since I can allocate an arbitrary number of pages, I think allocating
a rounded up amount of memory, so that it's multiple of PAGE_SIZE should
be enough.
But maybe I could do better than that:
a) support pre-allocation of x pages
b) define, as pool parameter, the minimum number of pages to allocate
every time there is a refill
c) both a and b
----
2. the flavor of page_frag from page_alloc relies on page->_refcount,
however neither vmap_area, nor vm_struct seem to have anything like
that. (My reasoning is that I should do the accounting not on page
level, but based on the virtual area that I get when I allocate new
memory) What would be the best way to do refcounting for the area?
a) use the the page->_refcount from the first page that belongs to the area
b) add the _refcount to either vm_struct or vmap_area (I am not really
sure of why these two structures exist as separate entities, rather than
a single one - cache optimization?)
----
3. I will have to add a list of chunks (in genalloc lingo, or areas, if
we refer to the new implementation), because I will still need to
iterate over all the memory that belongs to a pool, for either write
protecting it or for destroying the pool. I have two options:
a) handle the chunks within the pmalloc pool
b) create an intermediate type of pool (vfrag_pool?) and then include it
in the pmalloc pool structure.
I'd lean toward option a, but I thought I might as well ask for advice
before I implement the less desirable option (whatever it might be).
--
thanks, igor
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