Re: [PATCH 4/3 v2] x86/mm/doc: Enhance the x86-64 virtual memory layout descriptions

[Baoquan He]

Hi Andy, Ingo

On 10/06/18 at 03:17pm, Andy Lutomirski wrote:
> On Sat, Oct 6, 2018 at 10:03 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > ... but unless I'm missing something it's not really fundamental for it to be at the PGD level
> > - it could be two levels lower as well, and it could move back to the same place where it's on
> > the 47-bit kernel.
> >
> 
> The subtlety is that, if it's lower than the PGD level, there end up
> being some tables that are private to each LDT-using mm that map
> things other than the LDT.  Those tables cover the same address range
> as some corresponding tables in init_mm, and if those tables in
> init_mm change after the LDT mapping is set up, the changes won't
> propagate.
> 
> So it probably could be made to work, but it would take some extra care.

I didn't know LDT well before, after some investigation, seems mainly
user space program like Wine will use it to protect/isolate something
by calling modify_ldt syscall, and Xen also use it. still I don't know
how they will use it to manipulate code/data segments.

While from the current kernel code, it can contains array of 8192 entries,
and each entry is 8 Byte, when PTI not enabled. If PTI is enabled, it's
doubled, 2 slots to map, 2 * 8192 * 8, 128KB in all. So one pmd entry can
cover it.

In 4-level paging mode, we reserve 512 GB virtual address space for it to
map, the 512 GB is one PGD entry. In 5-level paging mode, we reserve 4
PB for mapping LDT, and leave the previous 512 GB space next to
cpu_entry_area mapping empty as unused hole. Maybe we can still put LDT
map for PTI in the old place, after cpu_entry_area mapping in 5-level.
Then in 5-level, 512 GB is only one p4d entry, however it's in the last
pgd entry, each pgd points to 256 TB area, and the last pgd entry will
points to p4d table which always exists in system since it contains
kernel text mapping etc. Now if LDT take one entry in the always
existing p4d table, maybe it can still works as before it owns a whole
pgd entry, oh, no, 4 PB will cost 16 pgd entries.

Most importantly, putting LDT map for PTI in KASLR area, won't it cause
code bug, if we randomize the direct mapping/vmaloc/vmemmap to make them
overlap with LDT map area? We didn't take LDT into consideration when do
memory region KASLR.


4-level virutal memory layout:

ffff800000000000 | -128    TB | ffff87ffffffffff |    8 TB | ... guard hole, also reserved for hypervisor
ffff880000000000 | -120    TB | ffffc7ffffffffff |   64 TB | direct mapping of all physical memory (page_offset_base)
ffffc80000000000 |  -56    TB | ffffc8ffffffffff |    1 TB | ... unused hole
ffffc90000000000 |  -55    TB | ffffe8ffffffffff |   32 TB | vmalloc/ioremap space (vmalloc_base)
ffffe90000000000 |  -23    TB | ffffe9ffffffffff |    1 TB | ... unused hole
ffffea0000000000 |  -22    TB | ffffeaffffffffff |    1 TB | virtual memory map (vmemmap_base)
ffffeb0000000000 |  -21    TB | ffffebffffffffff |    1 TB | ... unused hole
ffffec0000000000 |  -20    TB | fffffbffffffffff |   16 TB | KASAN shadow memory
fffffc0000000000 |   -4    TB | fffffdffffffffff |    2 TB | ... unused hole
                 |            |                  |         | vaddr_end for KASLR
fffffe0000000000 |   -2    TB | fffffe7fffffffff |  0.5 TB | cpu_entry_area mapping
fffffe8000000000 |   -1.5  TB | fffffeffffffffff |  0.5 TB | LDT remap for PTI
					^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^	
ffffff0000000000 |   -1    TB | ffffff7fffffffff |  0.5 TB | %esp fixup stacks

5-level virtual memory layout:

ff10000000000000 |  -60    PB | ff8fffffffffffff |   32 PB | direct mapping of all physical memory (page_offset_base)
ff90000000000000 |  -28    PB | ff9fffffffffffff |    4 PB | LDT remap for PTI
					^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ffa0000000000000 |  -24    PB | ffd1ffffffffffff | 12.5 PB | vmalloc/ioremap space (vmalloc_base)
ffd2000000000000 |  -11.5  PB | ffd3ffffffffffff |  0.5 PB | ... unused hole
ffd4000000000000 |  -11    PB | ffd5ffffffffffff |  0.5 PB | virtual memory map (vmemmap_base)
ffd6000000000000 |  -10.5  PB | ffdeffffffffffff | 2.25 PB | ... unused hole
ffdf000000000000 |   -8.25 PB | fffffdffffffffff |   ~8 PB | KASAN shadow memory
fffffc0000000000 |   -4    TB | fffffdffffffffff |    2 TB | ... unused hole
                 |            |                  |         | vaddr_end for KASLR
fffffe0000000000 |   -2    TB | fffffe7fffffffff |  0.5 TB | cpu_entry_area mapping
fffffe8000000000 |   -1.5  TB | fffffeffffffffff |  0.5 TB | ... unused hole
ffffff0000000000 |   -1    TB | ffffff7fffffffff |  0.5 TB | %esp fixup stacks