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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Tue, 3 Mar 2020 14:07:11 +0100
Message-ID: <CAKv+Gu82eEpZFz5Qto+BnKifM4duv8sBTx3YhLXU8ZPPsND+Rg@mail.gmail.com>
Subject: Re: [PATCH v2] lib/refcount: Document interaction with PID_MAX_LIMIT
To:     Jann Horn <jannh@google.com>
Cc:     Will Deacon <will@kernel.org>, Kees Cook <keescook@chromium.org>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Hanjun Guo <guohanjun@huawei.com>,
        Jan Glauber <jglauber@marvell.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
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On Tue, 3 Mar 2020 at 11:54, Jann Horn <jannh@google.com> wrote:
>
> Document the circumstances under which refcount_t's saturation mechanism
> works deterministically.
>
> Signed-off-by: Jann Horn <jannh@google.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
>