Re: [RFC][PATCH v2] procfs: Always expose /proc/<pid>/map_files/ and make it readable

[Calvin Owens <calvinowens@fb.com>]

On Monday 02/02 at 09:01 -0500, Austin S Hemmelgarn wrote:
> On 2015-01-30 20:58, Calvin Owens wrote:
> >On Thursday 01/29 at 17:30 -0800, Kees Cook wrote:
> >>On Tue, Jan 27, 2015 at 8:38 PM, Calvin Owens <calvinowens@fb.com> wrote:
> >>>On Monday 01/26 at 15:43 -0800, Andrew Morton wrote:
> >>>>On Tue, 27 Jan 2015 00:00:54 +0300 Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> >>>>
> >>>>>On Mon, Jan 26, 2015 at 02:47:31PM +0200, Kirill A. Shutemov wrote:
> >>>>>>On Fri, Jan 23, 2015 at 07:15:44PM -0800, Calvin Owens wrote:
> >>>>>>>Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and
> >>>>>>>is only exposed if CONFIG_CHECKPOINT_RESTORE is set. This interface
> >>>>>>>is very useful for enumerating the files mapped into a process when
> >>>>>>>the more verbose information in /proc/<pid>/maps is not needed.
> >>>>
> >>>>This is the main (actually only) justification for the patch, and it it
> >>>>far too thin.  What does "not needed" mean.  Why can't people just use
> >>>>/proc/pid/maps?
> >>>
> >>>The biggest difference is that if you do something like this:
> >>>
> >>>         fd = open("/stuff", O_BLAH);
> >>>         map = mmap(NULL, 4096, PROT_BLAH, MAP_SHARED, fd, 0);
> >>>         close(fd);
> >>>         unlink("/stuff");
> >>>
> >>>...then map_files/ gives you a way to get a file descriptor for
> >>>"/stuff", which you couldn't do with /proc/pid/maps.
> >>>
> >>>It's also something of a win if you just want to see what is mapped at a
> >>>specific address, since you can just readlink() the symlink for the
> >>>address range you care about and it will go grab the appropriate VMA and
> >>>give you the answer. /proc/pid/maps requires walking the VMA tree, which
> >>>is quite expensive for processes with many thousands of threads, even
> >>>without the O(N^2) issue.
> >>>
> >>>(You have to know what address range you want though, since readdir() on
> >>>map_files/ obviously has to walk the VMA tree just like /proc/N/maps.)
> >>>
> >>>>>>>This patch moves the folder out from behind CHECKPOINT_RESTORE, and
> >>>>>>>removes the CAP_SYS_ADMIN restrictions. Following the links requires
> >>>>>>>the ability to ptrace the process in question, so this doesn't allow
> >>>>>>>an attacker to do anything they couldn't already do before.
> >>>>>>>
> >>>>>>>Signed-off-by: Calvin Owens <calvinowens@fb.com>
> >>>>>>
> >>>>>>Cc +linux-api@
> >>>>>
> >>>>>Looks good to me, thanks! Though I would really appreciate if someone
> >>>>>from security camp take a look as well.
> >>>>
> >>>>hm, who's that.  Kees comes to mind.
> >>>>
> >>>>And reviewers' task would be a heck of a lot easier if they knew what
> >>>>/proc/pid/map_files actually does.  This:
> >>>>
> >>>>akpm3:/usr/src/25> grep -r map_files Documentation
> >>>>akpm3:/usr/src/25>
> >>>>
> >>>>does not help.
> >>>>
> >>>>The 640708a2cff7f81 changelog says:
> >>>>
> >>>>:     This one behaves similarly to the /proc/<pid>/fd/ one - it contains
> >>>>:     symlinks one for each mapping with file, the name of a symlink is
> >>>>:     "vma->vm_start-vma->vm_end", the target is the file.  Opening a symlink
> >>>>:     results in a file that point exactly to the same inode as them vma's one.
> >>>>:
> >>>>:     For example the ls -l of some arbitrary /proc/<pid>/map_files/
> >>>>:
> >>>>:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80403000-7f8f80404000 -> /lib64/libc-2.5.so
> >>>>:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f8061e000-7f8f80620000 -> /lib64/libselinux.so.1
> >>>>:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80826000-7f8f80827000 -> /lib64/libacl.so.1.1.0
> >>>>:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80a2f000-7f8f80a30000 -> /lib64/librt-2.5.so
> >>>>:      | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80a30000-7f8f80a4c000 -> /lib64/ld-2.5.so
> >>>>
> >>>>afacit this info is also available in /proc/pid/maps, so things
> >>>>shouldn't get worse if the /proc/pid/map_files permissions are at least
> >>>>as restrictive as the /proc/pid/maps permissions.  Is that the case?
> >>>>(Please add to changelog).
> >>>
> >>>Yes, the only difference is that you can follow the link as per above.
> >>>I'll resend with a new message explaining that and the deletion thing.
> >>>
> >>>>There's one other problem here: we're assuming that the map_files
> >>>>implementation doesn't have bugs.  If it does have bugs then relaxing
> >>>>permissions like this will create new vulnerabilities.  And the
> >>>>map_files implementation is surprisingly complex.  Is it bug-free?
> >>>
> >>>While I was messing with it I used it a good bit and didn't see any
> >>>issues, although I didn't actively try to fuzz it or anything. I'd be
> >>>happy to write something to test hammering it in weird ways if you like.
> >>>I'm also happy to write testcases for namespaces.
> >>>
> >>>So far as security issues, as others have pointed out you can't follow
> >>>the links unless you can ptrace the process in question, which seems
> >>>like a pretty solid guarantee. As Cyrill pointed out in the discussion
> >>>about the documentation, that's the same protection as /proc/N/fd/*, and
> >>>those links function in the same way.
> >>
> >>My concern here is that fd/* are connected as streams, and while that
> >>has a certain level of badness as an external-to-the-process attacker,
> >>PTRACE_MODE_READ is much weaker than PTRACE_MODE_ATTACH (which is
> >>required for access to /proc/N/mem). Since these fds are the things
> >>mapped into memory on a process, writing to them is a subset of access
> >>to /proc/N/mem, and I don't feel that PTRACE_MODE_READ is sufficient.
> >
> >If you haven't done close() on a mmapped file, doesn't fd/* allow the
> >same access to the corresponding regions of memory? Or am I missing
> >something?
> >
> >But that said, I can't think of any reason making it MODE_ATTACH would
> >be a problem. Would you rather that be enforced on follow_link() like
> >the original patch did, or enforce it for the whole directory?
> >
> >
> Whole directory would probably be better, as even just the mapped
> ranges could be considered sensitive information. 

You can already get the ranges that are mapped from /proc/N/maps with
PTRACE_MODE_READ, so that part isn't new information.

> Ideally, the check should be done on both follow_link(), and the
> directory itself.

Oh, I didn't mean restricting readdir(), I meant restricting any access
through the directory similar to how the original CAP_SYS_ADMIN check
was done.
 
Thanks,
Calvin