Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if file's opener may access task

Andy Lutomirski
2013-10-03T19:38:12Z

On Thu, Oct 3, 2013 at 12:29 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
> On Thu, Oct 03, 2013 at 04:12:37PM +0100, Andy Lutomirski wrote:
>> On Thu, Oct 3, 2013 at 3:36 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
>> > On Wed, Oct 02, 2013 at 05:44:17PM +0100, Andy Lutomirski wrote:
>> >> On Wed, Oct 2, 2013 at 3:55 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
>> >> > On Tue, Oct 01, 2013 at 06:36:34PM -0700, Andy Lutomirski wrote:
>> >> >> On 10/01/2013 01:26 PM, Djalal Harouni wrote:
>> >> >> > Since /proc entries varies at runtime, permission checks need to happen
>> >> >> > during each system call.
>> >> >> >
>> >> >> > However even with that /proc file descriptors can be passed to a more
>> >> >> > privileged process (e.g. a suid-exec) which will pass the classic
>> >> >> > ptrace_may_access() permission check. The open() call will be issued in
>> >> >> > general by an unprivileged process while the disclosure of sensitive
>> >> >> > /proc information will happen using a more privileged process at
>> >> >> > read(),write()...
>> >> >> >
>> >> >> > Therfore we need a more sophisticated check to detect if the cred of the
>> >> >> > process have changed, and if the cred of the original opener that are
>> >> >> > stored in the file->f_cred have enough permission to access the task's
>> >> >> > /proc entries during read(), write()...
>> >> >> >
>> >> >> > Add the proc_allow_access() function that will receive the file->f_cred
>> >> >> > as an argument, and tries to check if the opener had enough permission
>> >> >> > to access the task's /proc entries.
>> >> >> >
>> >> >> > This function should be used with the ptrace_may_access() check.
>> >> >> >
>> >> >> > Cc: Kees Cook <keescook@chromium.org>
>> >> >> > Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
>> >> >> > Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
>> >> >> > ---
>> >> >> > fs/proc/base.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> >> >> > fs/proc/internal.h | 2 ++
>> >> >> > 2 files changed, 58 insertions(+)
>> >> >> >
>> >> >> > diff --git a/fs/proc/base.c b/fs/proc/base.c
>> >> >> > index e834946..c29eeae 100644
>> >> >> > --- a/fs/proc/base.c
>> >> >> > +++ b/fs/proc/base.c
>> >> >> > @@ -168,6 +168,62 @@ int proc_same_open_cred(const struct cred *fcred)
>> >> >> > cap_issubset(cred->cap_permitted, fcred->cap_permitted));
>> >> >> > }
>> >> >> >
>> >> >> > +/* Returns 0 on success, -errno on denial. */
>> >> >> > +static int __proc_allow_access(const struct cred *cred,
>> >> >> > + struct task_struct *task, unsigned int mode)
>> >> >> > +{
>> >> >> > + int ret = 0;
>> >> >> > + const struct cred *tcred;
>> >> >> > + const struct cred *fcred = cred;
>> >> >> > +
>> >> >> > + rcu_read_lock();
>> >> >> > + tcred = __task_cred(task);
>> >> >> > + if (uid_eq(fcred->uid, tcred->euid) &&
>> >> >> > + uid_eq(fcred->uid, tcred->suid) &&
>> >> >> > + uid_eq(fcred->uid, tcred->uid) &&
>> >> >> > + gid_eq(fcred->gid, tcred->egid) &&
>> >> >> > + gid_eq(fcred->gid, tcred->sgid) &&
>> >> >> > + gid_eq(fcred->gid, tcred->gid))
>> >> >> > + goto out;
>> >> >> > +
>> >> >>
>> >> >> What's this for? Is it supposed to be an optimization? If so, it looks
>> >> >> potentially exploitable, although I don't really understand what you're
>> >> >> trying to do.
>> >> > This function should be used in addition to the ptrace_may_access() one.
>> >>
>> >> Sorry, I was unclear. I meant: what are the uid and gid checks for?
>> > The uid/gid are checks of the current (reader) on the target task, like
>> > the ptrace checks. fcred here is the cred of current at open time.
>> >
>>
>> This isn't a faithful copy of __ptrace_may_access -- the real function
>> gives LSMs a chance to veto ptracing. That's critical even without
>> LSMs because cap_ptrace_access_check needs to get called. (Think
>> about setcap'd programs instead of setuid programs.)
> Yes, I already did this, not only setuid, capabilities also are handled
> See the whole patch, please!
>
>
> Yes, and speaking about LSMs I've mentioned in my patches and doc, that
> the proposed function proc_allow_access() should be used after
> ptrace_may_access(). proc_allow_access() is not a replacement for
> ptrace_may_access(), it should be used *after* it.
>
> So cap_ptrace_access_check() is called, and before the file->f_cred
> checks. The LSM veto is already there.

It's possible that I've misunderstood your patches, but I really don't
see where you're calling into LSMs to give them a chance to veto
access by *f_cred*.

> 1) for proc_same_open_cred()
> if (f_cred->user_ns != cred->user_ns)
> return 0
>
> return (uid_eq(fcred->uid, cred->uid) &&
> gid_eq(fcred->gid, cred->gid) &&
> cap_issubset(cred->cap_permitted, f_cred->cap_permitted));
>
> So it handles the (1) of cap_ptrace_access_check()

No. This just means that, if there's a possibility that the caps are
wrong, you invoke ptrace_allow_access, which *does not re-check
capabilities*.

--Andy