Re: [PATCHv2 1/2] x86/mm: Move LDT remap out of KASLR region on 5-level paging

[Kirill A. Shutemov]

On Thu, Oct 25, 2018 at 10:18:09AM +0800, Baoquan He wrote:
> > We don't touch 4 pgd slot gap just before the direct mapping reserved
> > for a hypervisor, but move direct mapping by one slot instead.
> > 
> > The LDT mapping is per-mm, so we cannot move it into P4D page table next
> > to CPU_ENTRY_AREA without complicating PGD table allocation for 5-level
> > paging.
> 
> Here as discussed in private thread, at the first place you also agreed
> to put it in p4d entry next to CPU_ENTRY_AREA, but finally you changd
> mind, there must be some reasons when you implemented and investigated
> further to find out. Could you please say more about how it will
> complicating PGD table allocation for 5-level paging? Or give an use
> case where it will complicate?

On 5-level machine all memory starting from CPU_ENTRY_AREA (and part of
KASAN memory) is in the same P4D page table. All this memory is shared
across all processes, we just copy PGD entry -- all proceses point to the
same P4D page table. (I leave out PTI from the picture for simplicity.)

LDT is per-mm. If we would place it next to CPU_ENTRY_AREA we would need
to unshare P4D page table and create a new one on each fork and copy P4D
entries.

It's considerably more complex and would affect processes that never use
modify_ldt() at all.

Other option would be to move LDT remap *to* KASLR region for both paging
modes and make KALSR code aware about it: randomize it as we do for page
offset, vmalloc, vmap. It's probably better long term, but it's more
complex and I wanted to get backportable fix.

-- 
 Kirill A. Shutemov