futex: Prevent attaching to kernel threads

We happily allow userspace to declare a random kernel thread to be the
owner of a user space PI futex.

Found while analysing the fallout of Dave Jones syscall fuzzer.

We also should validate the thread group for private futexes and find
some fast way to validate whether the "alleged" owner has RW access on
the file which backs the SHM, but that's a separate issue.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Jones <davej@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <darren@dvhart.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Clark Williams <williams@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Carlos ODonell <carlos@redhat.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: http://lkml.kernel.org/r/20140512201701.194824402@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
futex: Validate atomic acquisition in futex_lock_pi_atomic()

We need to protect the atomic acquisition in the kernel against rogue
user space which sets the user space futex to 0, so the kernel side
acquisition succeeds while there is existing state in the kernel
associated to the real owner.

Verify whether the futex has waiters associated with kernel state.  If
it has, return -EINVAL.  The state is corrupted already, so no point in
cleaning it up.  Subsequent calls will fail as well.  Not our problem.

[ tglx: Use futex_top_waiter() and explain why we do not need to try
  	restoring the already corrupted user space state. ]

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Drewry <wad@chromium.org>
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
futex: Always cleanup owner tid in unlock_pi

If the owner died bit is set at futex_unlock_pi, we currently do not
cleanup the user space futex.  So the owner TID of the current owner
(the unlocker) persists.  That's observable inconsistant state,
especially when the ownership of the pi state got transferred.

Clean it up unconditionally.

Change-Id: I07e5828ef561e2d79670e6d22d7a2c5377be398c
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Drewry <wad@chromium.org>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
futex: Make lookup_pi_state more robust

The current implementation of lookup_pi_state has ambigous handling of
the TID value 0 in the user space futex.  We can get into the kernel
even if the TID value is 0, because either there is a stale waiters bit
or the owner died bit is set or we are called from the requeue_pi path
or from user space just for fun.

The current code avoids an explicit sanity check for pid = 0 in case
that kernel internal state (waiters) are found for the user space
address.  This can lead to state leakage and worse under some
circumstances.

Handle the cases explicit:

       Waiter | pi_state | pi->owner | uTID      | uODIED | ?

  [1]  NULL   | ---      | ---       | 0         | 0/1    | Valid
  [2]  NULL   | ---      | ---       | >0        | 0/1    | Valid

  [3]  Found  | NULL     | --        | Any       | 0/1    | Invalid

  [4]  Found  | Found    | NULL      | 0         | 1      | Valid
  [5]  Found  | Found    | NULL      | >0        | 1      | Invalid

  [6]  Found  | Found    | task      | 0         | 1      | Valid

  [7]  Found  | Found    | NULL      | Any       | 0      | Invalid

  [8]  Found  | Found    | task      | ==taskTID | 0/1    | Valid
  [9]  Found  | Found    | task      | 0         | 0      | Invalid
  [10] Found  | Found    | task      | !=taskTID | 0/1    | Invalid

 [1] Indicates that the kernel can acquire the futex atomically. We
     came came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.

 [2] Valid, if TID does not belong to a kernel thread. If no matching
     thread is found then it indicates that the owner TID has died.

 [3] Invalid. The waiter is queued on a non PI futex

 [4] Valid state after exit_robust_list(), which sets the user space
     value to FUTEX_WAITERS | FUTEX_OWNER_DIED.

 [5] The user space value got manipulated between exit_robust_list()
     and exit_pi_state_list()

 [6] Valid state after exit_pi_state_list() which sets the new owner in
     the pi_state but cannot access the user space value.

 [7] pi_state->owner can only be NULL when the OWNER_DIED bit is set.

 [8] Owner and user space value match

 [9] There is no transient state which sets the user space TID to 0
     except exit_robust_list(), but this is indicated by the
     FUTEX_OWNER_DIED bit. See [4]

[10] There is no transient state which leaves owner and user space
     TID out of sync.

Change-Id: I50f9a762f3e6d3d210cc88968188d423760fb69b
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Drewry <wad@chromium.org>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>