RE: [PATCH] mm/vmalloc: randomize vmalloc() allocations

[David Laight <David.Laight@ACULAB.COM>]

From: Topi Miettinen
> Sent: 04 December 2020 10:58
> 
> On 4.12.2020 1.15, David Laight wrote:
> > From: Mike Rapoport
> >> Sent: 03 December 2020 06:58
> >>
> >> On Wed, Dec 02, 2020 at 08:49:06PM +0200, Topi Miettinen wrote:
> >>> On 1.12.2020 23.45, Topi Miettinen wrote:
> >>>> Memory mappings inside kernel allocated with vmalloc() are in
> >>>> predictable order and packed tightly toward the low addresses. With
> >>>> new kernel boot parameter 'randomize_vmalloc=1', the entire area is
> >>>> used randomly to make the allocations less predictable and harder to
> >>>> guess for attackers.
> >
> > Isn't that going to horribly fragment the available address space
> > and make even moderate sized allocation requests fail (or sleep).
> 
> For 32 bit architecture this is a real issue, but I don't think for 64
> bits it will be a problem. You can't fragment the virtual memory space
> for small allocations because the resulting page tables will not fit in
> RAM for existing or near future systems.

Hmmm truly random allocations are going to need 3 or 4 extra page tables
on 64bit systems. A bit overhead for 4k allocates.
While you won't run out of address space, you will run out of memory.

Randomising the allocated address with the area that already
has page tables allocated might make a bit of sense.
Then allocate similar(ish) sized items from the same 'large' pages.

I was wondering if a flag indicating whether an allocate was 'long term'
or 'short term' might help the placement.
Short term small items could be used to fill the space in 'large pages' left
by non-aligned length large items.

Trouble is you need a CBU (Crystal Ball Unit) to get it right.

	David

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