Re: [PATCH -mm] mm, pagemap: Fix swap offset value for PMD migration entry

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Apr 10, 2018 at 08:57:19AM +0800, Huang, Ying wrote:
> >> the swap offset reported doesn't
> >> reflect this.  And in the loop to report information of each sub-page,
> >> the swap offset isn't increased accordingly as that for PFN.
> >> 
> >> BTW: migration swap entries have PFN information, do we need to
> >> restrict whether to show them?
> >
> > For what reason?  Address obfuscation?
> 
> This is an existing feature for PFN report of /proc/<pid>/pagemap,
> reason is in following commit log.  I am wondering whether that is
> necessary for migration swap entries too.
> 
> ab676b7d6fbf4b294bf198fb27ade5b0e865c7ce
> Author:     Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> AuthorDate: Mon Mar 9 23:11:12 2015 +0200
> Commit:     Linus Torvalds <torvalds@linux-foundation.org>
> CommitDate: Tue Mar 17 09:31:30 2015 -0700
> 
> pagemap: do not leak physical addresses to non-privileged userspace
> 
> As pointed by recent post[1] on exploiting DRAM physical imperfection,
> /proc/PID/pagemap exposes sensitive information which can be used to do
> attacks.
> 
> This disallows anybody without CAP_SYS_ADMIN to read the pagemap.
> 
> [1] http://googleprojectzero.blogspot.com/2015/03/exploiting-dram-rowhammer-bug-to-gain.html
> 
> [ Eventually we might want to do anything more finegrained, but for now
>   this is the simple model.   - Linus ]

Note that there's follow up to the commit: 

1c90308e7a77 ("pagemap: hide physical addresses from non-privileged users")

It introduces pm->show_pfn and it should be applied to swap entries too.

-- 
 Kirill A. Shutemov