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

Andy Lutomirski
2013-10-02T16:44:45Z

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?