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Message-ID: <202003021434.713F5559@keescook>
Date: Mon, 2 Mar 2020 14:37:46 -0800
From: Kees Cook <keescook@...omium.org>
To: Jann Horn <jannh@...gle.com>
Cc: Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
kernel list <linux-kernel@...r.kernel.org>,
Elena Reshetova <elena.reshetova@...el.com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Hanjun Guo <guohanjun@...wei.com>,
Jan Glauber <jglauber@...vell.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] lib/refcount: Document interaction with PID_MAX_LIMIT
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@...gle.com>
Acked-by: Kees Cook <keescook@...omium.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
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