openrisc: fix arbitrary kernel memory access via or1k_atomic syscallgrafted

sys_or1k_atomic() (syscall 244 in the "or1k" ABI) takes two user
pointers, v1 and v2, and swaps the words they point to in hand-written
assembly.

    l.lwz   r29,0(r4)
    l.lwz   r27,0(r5)
    l.sw    0(r4),r27
    l.sw    0(r5),r29

The pointers are not checked with access_ok(). The four memory
accesses also have no exception table entries.

A caller passes a kernel address as either pointer, and the syscall
reads from and writes to it directly.

This gives an unprivileged process a kernel read/write primitive. It
overwrites kernel data such as the sys_call_table, gaining code
execution in kernel context.

Check both pointers before entering the critical section. Add fixups
for the four memory accesses so faults on valid but unmapped user
addresses return -EFAULT.

[shorne@gmail.com: fix comment style]
Fixes: 9d02a4283e9c ("OpenRISC: Boot code")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>

1 files changed, 77 insertions, 0 deletions

diff --git a/Documentation/locking/rt-mutex.rst b/Documentation/locking/rt-mutex.rst
new file mode 100644
index 000000000..3b5097a38
--- /dev/null
+++ b/Documentation/locking/rt-mutex.rst
@@ -0,0 +1,77 @@
+==================================
+RT-mutex subsystem with PI support
+==================================
+
+RT-mutexes with priority inheritance are used to support PI-futexes,
+which enable pthread_mutex_t priority inheritance attributes
+(PTHREAD_PRIO_INHERIT). [See Documentation/locking/pi-futex.rst for more details
+about PI-futexes.]
+
+This technology was developed in the -rt tree and streamlined for
+pthread_mutex support.
+
+Basic principles:
+-----------------
+
+RT-mutexes extend the semantics of simple mutexes by the priority
+inheritance protocol.
+
+A low priority owner of a rt-mutex inherits the priority of a higher
+priority waiter until the rt-mutex is released. If the temporarily
+boosted owner blocks on a rt-mutex itself it propagates the priority
+boosting to the owner of the other rt_mutex it gets blocked on. The
+priority boosting is immediately removed once the rt_mutex has been
+unlocked.
+
+This approach allows us to shorten the block of high-prio tasks on
+mutexes which protect shared resources. Priority inheritance is not a
+magic bullet for poorly designed applications, but it allows
+well-designed applications to use userspace locks in critical parts of
+an high priority thread, without losing determinism.
+
+The enqueueing of the waiters into the rtmutex waiter tree is done in
+priority order. For same priorities FIFO order is chosen. For each
+rtmutex, only the top priority waiter is enqueued into the owner's
+priority waiters tree. This tree too queues in priority order. Whenever
+the top priority waiter of a task changes (for example it timed out or
+got a signal), the priority of the owner task is readjusted. The
+priority enqueueing is handled by "pi_waiters".
+
+RT-mutexes are optimized for fastpath operations and have no internal
+locking overhead when locking an uncontended mutex or unlocking a mutex
+without waiters. The optimized fastpath operations require cmpxchg
+support. [If that is not available then the rt-mutex internal spinlock
+is used]
+
+The state of the rt-mutex is tracked via the owner field of the rt-mutex
+structure:
+
+lock->owner holds the task_struct pointer of the owner. Bit 0 is used to
+keep track of the "lock has waiters" state:
+
+ ============ ======= ================================================
+ owner        bit0    Notes
+ ============ ======= ================================================
+ NULL         0       lock is free (fast acquire possible)
+ NULL         1       lock is free and has waiters and the top waiter
+		      is going to take the lock [1]_
+ taskpointer  0       lock is held (fast release possible)
+ taskpointer  1       lock is held and has waiters [2]_
+ ============ ======= ================================================
+
+The fast atomic compare exchange based acquire and release is only
+possible when bit 0 of lock->owner is 0.
+
+.. [1] It also can be a transitional state when grabbing the lock
+       with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
+       we need to set the bit0 before looking at the lock, and the owner may
+       be NULL in this small time, hence this can be a transitional state.
+
+.. [2] There is a small time when bit 0 is set but there are no
+       waiters. This can happen when grabbing the lock in the slow path.
+       To prevent a cmpxchg of the owner releasing the lock, we need to
+       set this bit before looking at the lock.
+
+BTW, there is still technically a "Pending Owner", it's just not called
+that anymore. The pending owner happens to be the top_waiter of a lock
+that has no owner and has been woken up to grab the lock.