linux-kernel - Re: [PATCH v2 4/8] exec: Allow load_misc_binary to call prepare

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Message-ID: <87sgfvoi8d.fsf@x220.int.ebiederm.org>
Date:   Tue, 19 May 2020 14:08:34 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Rob Landley <rob@...dley.net>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        linux-security-module@...r.kernel.org,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2 4/8] exec: Allow load_misc_binary to call prepare_binfmt unconditionally

Kees Cook <keescook@...omium.org> writes:

> 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()

Thank you.

>> 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@...omium.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).
> 		 */

I think that is more words saying less.

While I agree it might be better.  I don't see what your comment adds to
the understanding.  What do you see my comment not saying that is important?

Eric