Date: Thu, 28 Mar 2019 15:26:19 +0100 From: Oleg Nesterov <oleg@...hat.com> To: Joel Fernandes <joel@...lfernandes.org> Cc: Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, 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>, Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>, "Reshetova, Elena" <elena.reshetova@...el.com> Subject: Re: [PATCH] Convert struct pid count to refcount_t On 03/27, Joel Fernandes wrote: > > Also, based on Kees comment, I think it appears to me that get_pid and > put_pid can race in this way in the original code right? > > get_pid put_pid > > atomic_dec_and_test returns 1 > atomic_inc > kfree > > deref pid /* boom */ > ------------------------------------------------- > > I think get_pid needs to call atomic_inc_not_zero() No. get_pid() should only be used if you already have a reference or you do something like rcu_read_lock(); pid = find_vpid(); get_pid(); rcu_read_lock(); in this case we rely on call_rcu(delayed_put_pid) which drops the initial reference. If put_pid() sees pid->count == 1, then a) nobody else has a reference and b) nobody else can find this pid on rcu-protected lists, so it is safe to free it. Oleg.
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