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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 10 Jul 2021 11:21:34 -0700
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Subject: Re: [PATCH] procfs: Prevent unpriveleged processes accessing fdinfo
To:     Kalesh Singh <kaleshsingh@google.com>
Cc:     Kees Cook <keescook@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Christian Koenig <christian.koenig@amd.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Hridya Valsaraju <hridya@google.com>,
        Android Kernel Team <kernel-team@android.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Thu, Jul 8, 2021 at 8:57 AM Kalesh Singh <kaleshsingh@google.com> wrote:
>
> The file permissions on the fdinfo dir from were changed from
> S_IRUSR|S_IXUSR to S_IRUGO|S_IXUGO, and a PTRACE_MODE_READ check was
> added for opening the fdinfo files [1]. However, the ptrace permission
> check was not added to the directory, allowing anyone to get the open FD
> numbers by reading the fdinfo directory.
>
> Add the missing ptrace permission check for opening the fdinfo directory.

The more I look at this, the more I feel like we should look at
instead changing how "get_proc_task()" works.

That's one of the core functions for /proc, and I wonder if we
couldn't just make it refuse to look up a task that has gone through a
suid execve() since the proc inode was opened.

I don't think it's basically ever ok to open something for one thread,
and then use it after the thread has gone through a suid thing.

In fact, I wonder if we could make it even stricter, and go "any exec
at all", but I think a suid exec might be the minimum we should do.

Then the logic really becomes very simple: we did the permission
checks at open time (like UNIX permission checks should be done), and
"get_proc_task()" basically verifies that "yeah, that open-time
decision is still valid".

Wouldn't that make a lot of sense?

             Linus