Document the circumstances under which refcount_t's saturation mechanism works deterministically. Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> --- include/linux/refcount.h | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-) diff --git a/include/linux/refcount.h b/include/linux/refcount.h index 0ac50cf62d062..cf14db393d89d 100644 --- a/include/linux/refcount.h +++ b/include/linux/refcount.h @@ -38,11 +38,20 @@ * 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. + * 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 * =============== base-commit: 98d54f81e36ba3bf92172791eba5ca5bd813989b -- 2.25.0.265.gbab2e86ba0-goog
