locking/refcount: Document interaction with PID_MAX_LIMIT

Document the circumstances under which refcount_t's saturation mechanism
works deterministically.
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200303105427.260620-1-jannh@google.com

include/linux/refcount.h

@@ -38,11 +38,24 @@
  * atomic operations, then the count will continue to edge closer to 0. If it
  * reaches a value of 1 before /any/ of the threads reset it to the saturated
  * value, then a concurrent refcount_dec_and_test() may erroneously free the
- * underlying object. Given the precise timing details involved with the
- * round-robin scheduling of each thread manipulating the refcount and the need
- * to hit the race multiple times in succession, there doesn't appear to be a
- * practical avenue of attack even if using refcount_add() operations with
- * larger increments.
+ * underlying object.
+ * Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently
+ * 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK).
+ * With the current PID limit, if no batched refcounting operations are used and
+ * the attacker can't repeatedly trigger kernel oopses in the middle of refcount
+ * operations, this makes it impossible for a saturated refcount to leave the
+ * saturation range, even if it is possible for multiple uses of the same
+ * refcount to nest in the context of a single task:
+ *
+ *     (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT =
+ *     0x40000000 / 0x400000 = 0x100 = 256
+ *
+ * If hundreds of references are added/removed with a single refcounting
+ * operation, it may potentially be possible to leave the saturation range; but
+ * given the precise timing details involved with the round-robin scheduling of
+ * each thread manipulating the refcount and the need to hit the race multiple
+ * times in succession, there doesn't appear to be a practical avenue of attack
+ * even if using refcount_add() operations with larger increments.
  *
  * Memory ordering
  * ===============