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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id i25sm603978pgi.9.2020.09.23.12.04.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Sep 2020 12:04:09 -0700 (PDT) Date: Wed, 23 Sep 2020 12:04:08 -0700 From: Kees Cook To: Shuah Khan Cc: corbet@lwn.net, gregkh@linuxfoundation.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 01/11] counters: Introduce counter and counter_atomic Message-ID: <202009231152.5023C4656F@keescook> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Tue, Sep 22, 2020 at 07:43:30PM -0600, Shuah Khan wrote: > Introduce Simple atomic and non-atomic counters. > > There are a number of atomic_t usages in the kernel where atomic_t api > is used strictly for counting and not for managing object lifetime. In > some cases, atomic_t might not even be needed. Thank you for working on a counter API! I'm glad to see work here, though I have some pretty significant changes to request; see below... > > The purpose of these counters is twofold: 1. clearly differentiate > atomic_t counters from atomic_t usages that guard object lifetimes, > hence prone to overflow and underflow errors. It allows tools that scan > for underflow and overflow on atomic_t usages to detect overflow and > underflows to scan just the cases that are prone to errors. 2. provides > non-atomic counters for cases where atomic isn't necessary. > > Simple atomic and non-atomic counters api provides interfaces for simple > atomic and non-atomic counters that just count, and don't guard resource > lifetimes. Counters will wrap around to 0 when it overflows and should > not be used to guard resource lifetimes, device usage and open counts > that control state changes, and pm states. > > Using counter_atomic to guard lifetimes could lead to use-after free > when it overflows and undefined behavior when used to manage state > changes and device usage/open states. > > Signed-off-by: Shuah Khan I would really like these APIs to be _impossible_ to use for object lifetime management. To that end, I would like to have all of the *_return() functions removed. It should be strictly init, inc, dec, read. > +There are a number of atomic_t usages in the kernel where atomic_t api > +is used strictly for counting and not for managing object lifetime. In > +some cases, atomic_t might not even be needed. Why even force the distinction? I think all the counters should be atomic and then there is no chance they will get accidentally used in places where someone *thinks* it's safe to use a non-atomic. So, "_atomic" can be removed from the name and the non-atomic implementation can get removed. Anyone already using non-atomic counters is just using "int" and "long" anyway. Let's please only create APIs that are always safe to use, and provide some benefit over a native time. > +Simple atomic and non-atomic counters api provides interfaces for simple > +atomic and non-atomic counters that just count, and don't guard resource > +lifetimes. Counters will wrap around to 0 when it overflows and should > +not be used to guard resource lifetimes, device usage and open counts > +that control state changes, and pm states. > + > +Using counter_atomic to guard lifetimes could lead to use-after free > +when it overflows and undefined behavior when used to manage state > +changes and device usage/open states. > + > +Use refcnt_t interfaces for guarding resources. typo: refcount_t (this typo is repeated in a few places) > + > +.. warning:: > + Counter will wrap around to 0 when it overflows. > + Should not be used to guard resource lifetimes. > + Should not be used to manage device state and pm state. > + > +Test Counters Module and selftest > +--------------------------------- > + > +Please see :ref:`lib/test_counters.c ` for how to > +use these interfaces and also test them. > + > +Selftest for testing: > +:ref:`testing/selftests/lib/test_counters.sh ` > + > +Atomic counter interfaces > +========================= > + > +counter_atomic and counter_atomic_long types use atomic_t and atomic_long_t > +underneath to leverage atomic_t api, providing a small subset of atomic_t > +interfaces necessary to support simple counters. :: > + > + struct counter_atomic { atomic_t cnt; }; > + struct counter_atomic_long { atomic_long_t cnt; }; "Unsized" and "Long" are both unhelpful here. If it's unsized, that tells nothing about the counter size. And "long" changes with word size. I think counters should either _all_ be 64-bit, or they should be explicitly sized in their name. Either: struct counter; /* unsigned 64-bit, wraps back around to 0 */ or struct counter32; /* unsigned 32-bit, wraps back around to 0 */ struct counter64; /* unsigned 64-bit, wraps back around to 0 */ > --- /dev/null > +++ b/lib/test_counters.c > @@ -0,0 +1,283 @@ > +// SPDX-License-Identifier: GPL-2.0-only > +/* > + * Kernel module for testing Counters > + * > + * Authors: > + * Shuah Khan > + */ > + > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > + > +#include > +#include > + > +void test_counter_atomic(void) > +{ > + static struct counter_atomic acnt = COUNTER_ATOMIC_INIT(0); > + int start_val = counter_atomic_read(&acnt); > + int end_val; Please build this test using KUnit. > + start_val = counter_long_read(&acnt); > + end_val = counter_long_dec_return(&acnt); > + pr_info("Test read decrement and return: %ld to %ld - %s\n", > + start_val, end_val, > + ((start_val-1 == end_val) ? "PASS" : "FAIL")); I also see a lot of copy/paste patterns here. These should all use a common helper. -- Kees Cook