Re: [PATCHv2] x86/mm: Fix VDSO and VVAR placement on 5-level paging machines

[Edgecombe, Rick P]

On Thu, 2023-08-03 at 18:16 +0300, Kirill A. Shutemov wrote:
> Yingcong has noticed that on the 5-level paging machine, VDSO and
> VVAR
> VMAs are placed above the 47-bit border:
> 
> 8000001a9000-8000001ad000 r--p 00000000 00:00
> 0                          [vvar]
> 8000001ad000-8000001af000 r-xp 00000000 00:00
> 0                          [vdso]
> 
> This might confuse users who are not aware of 5-level paging and
> expect
> all userspace addresses to be under the 47-bit border.
> 
> So far problem has only been triggered with ASLR disabled, although
> it
> may also occur with ASLR enabled if the layout is randomized in a
> just
> right way.
> 
> The problem happens due to custom placement for the VMAs in the VDSO
> code: vdso_addr() tries to place them above the stack and checks the
> result against TASK_SIZE_MAX, which is wrong. TASK_SIZE_MAX is set to
> the 56-bit border on 5-level paging machines. Use DEFAULT_MAP_WINDOW
> instead.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reported-by: Yingcong Wu <yingcong.wu@xxxxxxxxx>
> Fixes: b569bab78d8d ("x86/mm: Prepare to expose larger address space
> to userspace")

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>