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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>,
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<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
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Trébuchet <philippe.trebuchet@....gouv.fr>, Scott Shell
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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
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