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Date: Wed, 27 Mar 2019 17:06:04 -0700
From: Kees Cook <keescook@...omium.org>
To: "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Android Kernel Team <kernel-team@...roid.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Andrew Morton <akpm@...ux-foundation.org>, "Eric W. Biederman" <ebiederm@...ssion.com>,
Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>, Oleg Nesterov <oleg@...hat.com>,
"Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: [PATCH] Convert struct pid count to refcount_t
On Wed, Mar 27, 2019 at 7:53 AM Joel Fernandes (Google)
<joel@...lfernandes.org> 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@...omium.org
> Cc: kernel-team@...roid.com
> Cc: kernel-hardening@...ts.openwall.com
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> [...]
> 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 <linux/init_task.h>
> #include <linux/syscalls.h>
> #include <linux/proc_ns.h>
> -#include <linux/proc_fs.h>
> +#include <linux/refcount.h>
> #include <linux/sched/task.h>
> #include <linux/idr.h>
>
> @@ -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
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