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Date: Tue, 3 Mar 2020 14:07:11 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Jann Horn <jannh@...gle.com>
Cc: Will Deacon <will@...nel.org>, Kees Cook <keescook@...omium.org>,
Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
kernel list <linux-kernel@...r.kernel.org>, Elena Reshetova <elena.reshetova@...el.com>,
Hanjun Guo <guohanjun@...wei.com>, Jan Glauber <jglauber@...vell.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2] lib/refcount: Document interaction with PID_MAX_LIMIT
On Tue, 3 Mar 2020 at 11:54, Jann Horn <jannh@...gle.com> wrote:
>
> Document the circumstances under which refcount_t's saturation mechanism
> works deterministically.
>
> Signed-off-by: Jann Horn <jannh@...gle.com>
I /think/ the main point of Kees's suggestion was that FUTEX_TID_MASK
is UAPI, so unlikely to change.
> ---
>
> Notes:
> v2:
> - write down the math (Kees)
>
> include/linux/refcount.h | 23 ++++++++++++++++++-----
> 1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index 0ac50cf62d062..0e3ee25eb156a 100644
> --- a/include/linux/refcount.h
> +++ b/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
> * ===============
>
> base-commit: 98d54f81e36ba3bf92172791eba5ca5bd813989b
> --
> 2.25.0.265.gbab2e86ba0-goog
>
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