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

Andy Lutomirski
2013-10-03T21:10:22Z

On Thu, Oct 3, 2013 at 1:13 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
> 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.

I don't want to put f_cred into this. I'd rather the patches just
check everything at open() time. Doing that will require some form of
revocation, I think.

Your current patches use f_cred, and they seem to do it wrong. So
please either fix it, stop using f_cred, or explain why it it's okay
despite not invoking LSM in the expected way.

--Andy