linux-kernel - Re: [PATCH v7 1/7] exec: Change uselib(2) IS

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Message-ID: <874kp9ngf0.fsf@x220.int.ebiederm.org>
Date:   Tue, 11 Aug 2020 14:14:43 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Eric Chiang <ericchiang@...gle.com>,
        Florian Weimer <fweimer@...hat.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Scott Shell <scottsh@...rosoft.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Thibaut Sautereau <thibaut.sautereau@...p-os.org>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v7 1/7] exec: Change uselib(2) IS_SREG() failure to EACCES

ebiederm@...ssion.com (Eric W. Biederman) writes:

> Mickaël Salaün <mic@...ikod.net> writes:
>
>> From: Kees Cook <keescook@...omium.org>
>>
>> Change uselib(2)' S_ISREG() error return to EACCES instead of EINVAL so
>> the behavior matches execve(2), and the seemingly documented value.
>> The "not a regular file" failure mode of execve(2) is explicitly
>> documented[1], but it is not mentioned in uselib(2)[2] which does,
>> however, say that open(2) and mmap(2) errors may apply. The documentation
>> for open(2) does not include a "not a regular file" error[3], but mmap(2)
>> does[4], and it is EACCES.
>
> Do you have enough visibility into uselib to be certain this will change
> will not cause regressions?
>
> My sense of uselib is that it would be easier to remove the system call
> entirely (I think it's last use was in libc5) than to validate that a
> change like this won't cause problems for the users of uselib.
>
> For the kernel what is important are real world users and the manpages
> are only important as far as they suggest what the real world users
> do.

Hmm.

My apologies. After reading the next patch I see that what really makes
this safe is: 73601ea5b7b1 ("fs/open.c: allow opening only regular files
during execve()").

As in practice this change has already been made and uselib simply
can not reach the !S_ISREG test.

It might make sense to drop this patch or include that reference
in the next posting of this patch.

Eric