From: Will Deacon <will@kernel.org>
To: Jann Horn <jannh@google.com>
Cc: 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>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Hanjun Guo <guohanjun@huawei.com>,
Jan Glauber <jglauber@marvell.com>,
Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v2] lib/refcount: Document interaction with PID_MAX_LIMIT
Date: Tue, 17 Mar 2020 22:27:18 +0000 [thread overview]
Message-ID: <20200317222717.GF20788@willie-the-truck> (raw)
In-Reply-To: <20200303105427.260620-1-jannh@google.com>
On Tue, Mar 03, 2020 at 11:54:27AM +0100, Jann Horn wrote:
> Document the circumstances under which refcount_t's saturation mechanism
> works deterministically.
>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>
> 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
Acked-by: Will Deacon <will@kernel.org>
Peter -- would you be able to take this through -tip, please?
Will
next prev parent reply other threads:[~2020-03-17 22:27 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-03 10:54 [PATCH v2] lib/refcount: Document interaction with PID_MAX_LIMIT Jann Horn
2020-03-03 13:07 ` Ard Biesheuvel
2020-03-03 13:57 ` Jann Horn
2020-03-03 21:09 ` Kees Cook
2020-03-17 22:27 ` Will Deacon [this message]
2020-03-24 15:29 ` Peter Zijlstra
2020-04-08 12:20 ` [tip: locking/urgent] locking/refcount: " tip-bot2 for Jann Horn
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