Re: [PATCH v2 6/9] procfs: add permission checks on the file's opener of /proc/*/stat

Andy Lutomirski
2013-10-02T16:46:43Z

On Wed, Oct 2, 2013 at 4:14 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
> On Tue, Oct 01, 2013 at 06:39:00PM -0700, Andy Lutomirski wrote:
>> On 10/01/2013 01:26 PM, Djalal Harouni wrote:
>> > Some fields of the /proc/*/stat are sensitive fields that need
>> > appropriate protection.
>> >
>> > However, /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 during read().
>> >
>> > To prevent it, use proc_same_open_cred() to detect if current's cred
>> > have changed between ->open() and ->read(), if so, call
>> > proc_allow_access() to check if the original file's opener had enough
>> > permissions to read these sensitive fields. This will prevent passing
>> > file descriptors to a more privileged process to leak data.
>> >
>> > The patch also adds a previously missing signal->cred_guard_mutex lock.
>> >
>> > This patch does not break userspace since it only hides the fields that
>> > were supposed to be protected.
>> >
>> > Cc: Kees Cook <keescook@chromium.org>
>> > Cc: Eric W. Biederman <ebiederm@xmission.com>
>> > Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
>> > ---
>> > fs/proc/array.c | 16 ++++++++++++++--
>> > 1 file changed, 14 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/fs/proc/array.c b/fs/proc/array.c
>> > index cbd0f1b..f034e05 100644
>> > --- a/fs/proc/array.c
>> > +++ b/fs/proc/array.c
>> > @@ -394,7 +394,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>> > char state;
>> > pid_t ppid = 0, pgid = -1, sid = -1;
>> > int num_threads = 0;
>> > - int permitted;
>> > + int permitted = 0;
>> > struct mm_struct *mm;
>> > unsigned long long start_time;
>> > unsigned long cmin_flt = 0, cmaj_flt = 0;
>> > @@ -404,10 +404,22 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>> > unsigned long rsslim = 0;
>> > char tcomm[sizeof(task->comm)];
>> > unsigned long flags;
>> > + struct file *file = m->private;
>> > + int same_cred = proc_same_open_cred(file->f_cred);
>> > + unsigned int ptrace_mode = PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT;
>> >
>> > state = *get_task_state(task);
>> > vsize = eip = esp = 0;
>> > - permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT);
>> > +
>> > + if (!mutex_lock_killable(&task->signal->cred_guard_mutex)) {
>> > + permitted = ptrace_may_access(task, ptrace_mode);
>> > + if (permitted && !same_cred)
>> > + permitted = proc_allow_access(file->f_cred,
>> > + task, ptrace_mode);
>> > +
>> > + mutex_unlock(&task->signal->cred_guard_mutex);
>> > + }
>> > +
>>
>> else permitted = false?
> permitted is initialized to 0

Never mind, then -- I read that wrong...

>
> First the original ptrace_may_access() check did not hold
> cred_guard_mutex, so add it. If we can't grab mutex then let permitted
> to be zero. Yes this a change in behaviour and I think it's correct, IOW
> we were not able to perform the ptrace_may_access() check, otherwise
> permitted will depend on checks result.
>
> However, there is still a race here since we set the permitted value
> before gathering the appropriate info about task. At the read() data moment
> this target task may have been gone privileged... , acquiring an X lock
> on target task, will just break/slow things, as it has been shown before...
> Not to mention that the race window is small...
>
>
>> But surely this would be *much* more comprehensible if you had
>> proc_allow_access do the entire check.
> I don't understand what you mean by "do the entire check" ?

I mean to move the entire "check current->cred and f_cred" check into
its own function rather than duplicating it at each call site.

--Andy