linux-kernel - Re: [PATCH] docs: procs -- Describe /proc/<pid>/map

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Message-ID: <CAGXu5jKR0HLqT=z+BwuVbmMmtKEQnVH2JMhLmmk9ojpGZPfnkw@mail.gmail.com>
Date:	Thu, 29 Jan 2015 17:31:12 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Calvin Owens <calvinowens@...com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Peter Feiner <pfeiner@...gle.com>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [PATCH] docs: procs -- Describe /proc/<pid>/map_files entry

On Tue, Jan 27, 2015 at 1:20 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Tue, Jan 27, 2015 at 11:50:49AM -0800, Kees Cook wrote:
>> > +
>> > +The main purpose of map_files directory is to be able to retrieve a set of
>> > +memory mapped files in a fast way instead of parsing /proc/<pid>/maps or
>> > +/proc/<pid>/smaps which contain a way more records. Same time one can open(2)
>> > +mappings from the listings of two processes and comparing inodes figure out
>> > +which anonymous memory areas are actually shared.
>>
>> Thanks for details! I still don't understand how this is used for
>> checkpoint/restore when the mmap offset isn't shown. Can't a process
>> map, say 4K of a file, from different offsets, and it would show up
>> as:
>>
>> 400000-401000 -> /some/file
>> 401000-402000 -> /some/file
>>
>> but there'd be no way to know how to restore that mapping?
>
> In criu we use a few sources of information (ie we scan not only
> map_files, but have to use /proc/pid/smaps as well which has
> offset for mapping). So at the end we have all picture under
> our hands.
>
>> Are these symlinks "regular" symlinks, or are they something more
>> special that bypasses VFS? If it bypasses VFS, I think adding and open
>> check with PTRACE_ATTACH is needed, since now you're able to _modify_
>> the memory space of the target process instead of just reading it.
>
> Opening them goes same way as open of /proc/pid/fd/ entries as
> far as I can tell. This should be enough, or I miss something
> obvious here? Otherwise opening /proc/pid/fd/ should use
> PTRACE_ATTACH instead of PTRACE_MODE_READ (as in proc_fd_access_allowed).

I wouldn't object to enhancing the check to ATTACH from READ, but I
worry what would break on the existing interface.

-Kees

-- 
Kees Cook
Chrome OS Security
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