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Date:   Thu, 30 May 2019 14:34:53 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Jann Horn <jannh@google.com>
Cc:     "Eric W . Biederman" <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        David Howells <dhowells@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()
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On 05/29, Jann Horn wrote:
>
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -450,6 +450,15 @@ int commit_creds(struct cred *new)
>  		if (task->mm)
>  			set_dumpable(task->mm, suid_dumpable);
>  		task->pdeath_signal = 0;
> +		/*
> +		 * If a task drops privileges and becomes nondumpable,
> +		 * the dumpability change must become visible before
> +		 * the credential change; otherwise, a __ptrace_may_access()
> +		 * racing with this change may be able to attach to a task it
> +		 * shouldn't be able to attach to (as if the task had dropped
> +		 * privileges without becoming nondumpable).
> +		 * Pairs with a read barrier in __ptrace_may_access().
> +		 */
>  		smp_wmb();

Hmm. Now that I tried to actually read this patch I do not understand this wmb().

commit_creds() does rcu_assign_pointer(real_cred) which implies smp_store_release(),
the dumpability change must be visible before ->real_cred is updated without any
additional barriers?

Oleg.