From: Topi Miettinen

Sent: 04 December 2020 10:58

On 4.12.2020 1.15, David Laight wrote:

quoted

From: Mike Rapoport

quoted

Sent: 03 December 2020 06:58

On Wed, Dec 02, 2020 at 08:49:06PM +0200, Topi Miettinen wrote:

quoted

On 1.12.2020 23.45, Topi Miettinen wrote:

quoted

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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