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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra \(Intel\)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Howells <dhowells@redhat.com>,
        James Morris <jamorris@linux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian Kellner <christian@kellner.me>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
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        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        "linux-mm\@kvack.org" <linux-mm@kvack.org>,
        "stable\@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-api\@vger.kernel.org" <linux-api@vger.kernel.org>
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Date:   Tue, 10 Mar 2020 14:01:15 -0500
In-Reply-To: <AM6PR03MB517057A2269C3A4FB287B76EE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com>
        (Bernd Edlinger's message of "Tue, 10 Mar 2020 18:45:11 +0100")
Message-ID: <87k13svxtw.fsf@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH 1/4] kernel/kcmp.c: Use new infrastructure to fix deadlocks in execve
Sender: linux-kernel-owner@vger.kernel.org
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Bernd Edlinger <bernd.edlinger@hotmail.de> writes:

> This changes kcmp_epoll_target to use the new exec_update_mutex
> instead of cred_guard_mutex.
>
> This should be safe, as the credentials are only used for reading,
> and furthermore ->mm and ->sighand are updated on execve,
> but only under the new exec_update_mutex.
>

Can you add a comment that the exec_update_mutex is not needed for
KCMP_FILE?  As both sets of credentials during exec are valid
for accessing the files so exec_update_mutex does not matter.

I don't think exec_update_mutex is needed for KCMP_SYSVSEM
or KCMP_EPOLL_TFD either.  As I don't think exec changes either
one of those.

Eric


> Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
> ---
>  kernel/kcmp.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/kcmp.c b/kernel/kcmp.c
> index a0e3d7a..b3ff928 100644
> --- a/kernel/kcmp.c
> +++ b/kernel/kcmp.c
> @@ -173,8 +173,8 @@ static int kcmp_epoll_target(struct task_struct *task1,
>  	/*
>  	 * One should have enough rights to inspect task details.
>  	 */
> -	ret = kcmp_lock(&task1->signal->cred_guard_mutex,
> -			&task2->signal->cred_guard_mutex);
> +	ret = kcmp_lock(&task1->signal->exec_update_mutex,
> +			&task2->signal->exec_update_mutex);
>  	if (ret)
>  		goto err;
>  	if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
> @@ -229,8 +229,8 @@ static int kcmp_epoll_target(struct task_struct *task1,
>  	}
>  
>  err_unlock:
> -	kcmp_unlock(&task1->signal->cred_guard_mutex,
> -		    &task2->signal->cred_guard_mutex);
> +	kcmp_unlock(&task1->signal->exec_update_mutex,
> +		    &task2->signal->exec_update_mutex);
>  err:
>  	put_task_struct(task1);
>  	put_task_struct(task2);