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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 17 Nov 2016 15:29:43 -0800
Message-ID: <CALCETrUvKpRCXRE+K512E_q9-o8Gzgb+3XsAzSo+ZFdgqeX-eQ@mail.gmail.com>
Subject: Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an
 unreadable file
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linux Containers <containers@lists.linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>, Jann Horn <jann@thejh.net>,
        Willy Tarreau <w@1wt.eu>, Kees Cook <keescook@chromium.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Thu, Nov 17, 2016 at 9:08 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
>
> It is the reasonable expectation that if an executable file is not
> readable there will be no way for a user without special privileges to
> read the file.  This is enforced in ptrace_attach but if we are
> already attached there is no enforcement if a readonly executable
> is exec'd.
>
> Therefore do the simple thing and if there is a non-dumpable
> executable that we are tracing without privilege fail to exec it.
>
> Fixes: v1.0
> Cc: stable@vger.kernel.org
> Reported-by: Andy Lutomirski <luto@amacapital.net>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>  fs/exec.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index fdec760bfac3..de107f74e055 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1230,6 +1230,11 @@ int flush_old_exec(struct linux_binprm * bprm)
>  {
>         int retval;
>
> +       /* Fail if the tracer can't read the executable */
> +       if ((bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP) &&
> +           !ptracer_capable(current, bprm->mm->user_ns))
> +               return -EPERM;
> +

At the very least, I think that BINPRM_FLAGS_ENFORCE_NONDUMP needs to
check capable_wrt_inode_uidgid too.  Otherwise we risk breaking:

$ gcc whatever.c
$ chmod 400 a.out
$ strace a.out