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

On Mon, Jan 26, 2015 at 3:43 PM, Andrew Morton
[off-list ref] wrote:

On Tue, 27 Jan 2015 00:00:54 +0300 Cyrill Gorcunov [off-list ref] wrote:

quoted

On Mon, Jan 26, 2015 at 02:47:31PM +0200, Kirill A. Shutemov wrote:

quoted

On Fri, Jan 23, 2015 at 07:15:44PM -0800, Calvin Owens wrote:

quoted

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?

quoted

quoted

quoted

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 <redacted>

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

If akpm's comments weren't clear: this needs to be fixed. Everything
in /proc should appear in 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

How is mmap offset represented in this output?

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).

Both maps and map_files uses ptrace_may_access (via mm_acces) with
PTRACE_MODE_READ, so I'm happy from a info leak perspective.

Are mount namespaces handled in this output?

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?

-Kees

-- 
Kees Cook
Chrome OS Security