linux-kernel - Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()

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Date:   Thu, 8 Oct 2020 18:23:00 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Topi Miettinen <toiwoton@...il.com>,
        linux-hardening@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()

On Thu, Oct 08, 2020 at 07:13:51PM +0200, Jann Horn wrote:
> You may want to consider whether it would be better to store
> information about free memory per subtree in the VMA tree, together
> with the maximum gap size that is already stored in each node, and
> then walk down the tree randomly, with the randomness weighted by free
> memory in the subtrees, but ignoring subtrees whose gaps are too
> small.

Please, no.  We're trying to get rid of the rbtree, not enhance it
further.  The new data structure is a B-tree and we'd rather not burden
it with extra per-node information (... although if we have to, we could)

> And for expanding stacks, it might be a good idea for other
> reasons as well (locking consistency) to refactor them such that the
> size in the VMA tree corresponds to the maximum expansion of the stack
> (and if an allocation is about to fail, shrink such stack mappings).

We're doing that as part of the B-tree ;-)  Although not the shrink
stack mappings part ...