[PATCH] lib/refcount: Document interaction with PID_MAX_LIMIT

[PATCH] lib/refcount: Document interaction with PID_MAX_LIMIT

[Jann Horn]

Document the circumstances under which refcount_t's saturation mechanism
works deterministically.

Signed-off-by: Jann Horn <jannh@google.com>
---
 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

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

[Kees Cook]

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

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

[Jann Horn]

On Mon, Mar 2, 2020 at 11:37 PM Kees Cook <keescook@chromium.org> wrote:
> 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)."

The value of that thing has changed three times in git history, and
there is a comment in threads.h that refers to it as being 0x1fffffff;
so I'm a bit hesitant to copy that around further.

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

Hm, I suppose... I'll send a v2.

> -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.