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Date:   Tue, 19 May 2020 11:27:25 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Greg Ungerer <gerg@linux-m68k.org>,
        Rob Landley <rob@landley.net>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        linux-security-module@vger.kernel.org,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH v2 4/8] exec: Allow load_misc_binary to call
 prepare_binfmt unconditionally
Message-ID: <202005191122.0A1FD07@keescook>
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On Mon, May 18, 2020 at 07:31:51PM -0500, Eric W. Biederman wrote:
> 
> Add a flag preserve_creds that binfmt_misc can set to prevent
> credentials from being updated.  This allows binfmt_misc to always
> call prepare_binfmt.  Allowing the credential computation logic to be

typo: prepare_binprm()

> consolidated.
> 
> Not replacing the credentials with the interpreters credentials is
> safe because because an open file descriptor to the executable is
> passed to the interpreter.   As the interpreter does not need to
> reopen the executable it is guaranteed to see the same file that
> exec sees.

Yup, looks good. Note below on comment.

Reviewed-by: Kees Cook <keescook@chromium.org>

> [...]
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index 8605ab4a0f89..dbb5614d62a2 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -26,6 +26,8 @@ struct linux_binprm {
>  	unsigned long p; /* current top of mem */
>  	unsigned long argmin; /* rlimit marker for copy_strings() */
>  	unsigned int
> +		/* It is safe to use the creds of a script (see binfmt_misc) */
> +		preserve_creds:1,

How about:

		/*
		 * A binfmt handler will set this to True before calling
		 * prepare_binprm() if it is safe to reuse the previous
		 * credentials, based on bprm->file (see binfmt_misc).
		 */

-- 
Kees Cook