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

[Djalal Harouni <tixxdz@opendz.org>]

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.

-- 
Djalal Harouni
http://opendz.org