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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 14 Jun 2021 11:02:46 -0700
Message-ID: <CAHk-=whLBq63v_h71YuBM2aNjCSBprkBEO3fevbWnkp8TDeh5g@mail.gmail.com>
Subject: Re: [PATCH] proc: Track /proc/$pid/attr/ opener mm_struct
To:     Kees Cook <keescook@chromium.org>
Cc:     youling257 <youling257@gmail.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Andrea Righi <andrea.righi@canonical.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        stable <stable@vger.kernel.org>, regressions@lists.linux.dev,
        LSM List <linux-security-module@vger.kernel.org>,
        Paul Moore <paul@paul-moore.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        SElinux list <selinux@vger.kernel.org>
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On Mon, Jun 14, 2021 at 9:45 AM Kees Cook <keescook@chromium.org> wrote:
>
>         /* A task may only write when it was the opener. */
> -       if (file->private_data != current->mm)
> +       if (!file->private_data || file->private_data != current->mm)

I don't think this is necessary.

If file->private_data is NULL, then the old test for private_data !=
current->mm will still work just fine.

Because if you can fool kernel threads to do the write for you, you
have bigger security issues than that test.

               Linus