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[209.85.217.48]) by smtp.gmail.com with ESMTPSA id r23sm18577294vke.36.2019.03.27.17.06.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 27 Mar 2019 17:06:16 -0700 (PDT) Received: by mail-vs1-f48.google.com with SMTP id j184so11067407vsd.11 for ; Wed, 27 Mar 2019 17:06:15 -0700 (PDT) X-Received: by 2002:a67:89c9:: with SMTP id l192mr23612585vsd.188.1553731575610; Wed, 27 Mar 2019 17:06:15 -0700 (PDT) MIME-Version: 1.0 References: <20190327145331.215360-1-joel@joelfernandes.org> In-Reply-To: <20190327145331.215360-1-joel@joelfernandes.org> From: Kees Cook Date: Wed, 27 Mar 2019 17:06:04 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] Convert struct pid count to refcount_t To: "Joel Fernandes (Google)" Cc: LKML , Android Kernel Team , Kernel Hardening , Andrew Morton , "Eric W. Biederman" , Matthew Wilcox , Michal Hocko , Oleg Nesterov , "Reshetova, Elena" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 27, 2019 at 7:53 AM Joel Fernandes (Google) wrote: > > struct pid's count is an atomic_t field used as a refcount. Use > refcount_t for it which is basically atomic_t but does additional > checking to prevent use-after-free bugs. No change in behavior if > CONFIG_REFCOUNT_FULL=n. > > Cc: keescook@chromium.org > Cc: kernel-team@android.com > Cc: kernel-hardening@lists.openwall.com > Signed-off-by: Joel Fernandes (Google) > [...] > diff --git a/kernel/pid.c b/kernel/pid.c > index 20881598bdfa..2095c7da644d 100644 > --- a/kernel/pid.c > +++ b/kernel/pid.c > @@ -37,7 +37,7 @@ > #include > #include > #include > -#include > +#include > #include > #include > > @@ -106,8 +106,8 @@ void put_pid(struct pid *pid) > return; > > ns = pid->numbers[pid->level].ns; > - if ((atomic_read(&pid->count) == 1) || > - atomic_dec_and_test(&pid->count)) { > + if ((refcount_read(&pid->count) == 1) || > + refcount_dec_and_test(&pid->count)) { Why is this (and the original code) safe in the face of a race against get_pid()? i.e. shouldn't this only use refcount_dec_and_test()? I don't see this code pattern anywhere else in the kernel. Beyond that, yes please. :) -Kees > kmem_cache_free(ns->pid_cachep, pid); > put_pid_ns(ns); > } > @@ -210,7 +210,7 @@ struct pid *alloc_pid(struct pid_namespace *ns) > } > > get_pid_ns(ns); > - atomic_set(&pid->count, 1); > + refcount_set(&pid->count, 1); > for (type = 0; type < PIDTYPE_MAX; ++type) > INIT_HLIST_HEAD(&pid->tasks[type]); > > -- > 2.21.0.392.gf8f6787159e-goog > -- Kees Cook