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

Djalal Harouni
2013-10-03T20:13:43Z

On Thu, Oct 03, 2013 at 12:37:49PM -0700, Andy Lutomirski wrote:
> 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*.
Ahh ok, I see, but why you want absolutly to put *f_cred* in this ?

That's not its job, LSM veto is handled during read() correctly before
proc_allow_access() and f_cred check. And if you want to do it correctly
then f_cred should be handled during its time, during ->open().
The correct way to handle it: ptrace_may_access() during ->open() and
each syscall for sensitive files.

Why add and speak about all this complexity where the correct check is
just add ptrace_may_access() during ->open() ? using *f_cred* in this
context and bring it here is not a valid argument IMO.

>
> > 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*.
Ohh Andy, we do check capabilities, please Andy are you looking to the
code ?

If the uid/gid match fails we do check capability, if it succeed why we
would check them ? same user!

+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;
+
+ if (mode & PTRACE_MODE_NOAUDIT)
+ ret = security_capable_noaudit(fcred, tcred->user_ns,
+ CAP_SYS_PTRACE);
+ else
+ ret = security_capable(fcred, tcred->user_ns,
+ CAP_SYS_PTRACE);
+
+out:
+ rcu_read_unlock();
+ return !ret ? ret : -EPERM;
+}

The patch was posted, this is a re-post!

> --Andy

--
Djalal Harouni