Re: [PATCH] lib/refcount: Document interaction with PID_MAX_LIMIT

[Kees Cook <keescook@chromium.org>]

On Mon, Mar 02, 2020 at 08:53:52PM +0100, Jann Horn wrote:
> Document the circumstances under which refcount_t's saturation mechanism
> works deterministically.
> 
> Signed-off-by: Jann Horn <jannh@google.com>

Acked-by: Kees Cook <keescook@chromium.org>

With one note below...

> ---
>  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).

Maybe just to clarify and make readers not have to go search the source:

	"... beyond FUTEX_TID_MASK, which is UAPI defined as 0x3fffffff)."

and is it worth showing the math on this, just to have it clearly
stated?

-Kees

> + * 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
> 

-- 
Kees Cook