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Date: Wed, 22 Jul 2020 21:04:28 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Thibaut Sautereau <thibaut.sautereau@...p-os.org>,
Kees Cook <keescook@...omium.org>
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>,
Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox <willy@...radead.org>,
Michael Kerrisk <mtk.manpages@...il.com>,
Mickaël Salaün <mickael.salaun@....gouv.fr>,
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@....gouv.fr>,
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 v6 5/7] fs,doc: Enable to enforce noexec mounts or file
exec through O_MAYEXEC
On 22/07/2020 18:16, Thibaut Sautereau wrote:
> On Thu, Jul 16, 2020 at 04:39:14PM +0200, Mickaël Salaün wrote:
>>
>> On 15/07/2020 22:37, Kees Cook wrote:
>>> On Tue, Jul 14, 2020 at 08:16:36PM +0200, Mickaël Salaün wrote:
>>>> @@ -2849,7 +2855,7 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>>>> case S_IFLNK:
>>>> return -ELOOP;
>>>> case S_IFDIR:
>>>> - if (acc_mode & (MAY_WRITE | MAY_EXEC))
>>>> + if (acc_mode & (MAY_WRITE | MAY_EXEC | MAY_OPENEXEC))
>>>> return -EISDIR;
>>>> break;
>>>
>>> (I need to figure out where "open for reading" rejects S_IFDIR, since
>>> it's clearly not here...)
>
> Doesn't it come from generic_read_dir() in fs/libfs.c?
>
>>>
>>>> case S_IFBLK:
>>>> @@ -2859,13 +2865,26 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>>>> fallthrough;
>>>> case S_IFIFO:
>>>> case S_IFSOCK:
>>>> - if (acc_mode & MAY_EXEC)
>>>> + if (acc_mode & (MAY_EXEC | MAY_OPENEXEC))
>>>> return -EACCES;
>>>> flag &= ~O_TRUNC;
>>>> break;
>>>
>>> This will immediately break a system that runs code with MAY_OPENEXEC
>>> set but reads from a block, char, fifo, or socket, even in the case of
>>> a sysadmin leaving the "file" sysctl disabled.
>>
>> As documented, O_MAYEXEC is for regular files. The only legitimate use
>> case seems to be with pipes, which should probably be allowed when
>> enforcement is disabled.
>
> By the way Kees, while we fix that for the next series, do you think it
> would be relevant, at least for the sake of clarity, to add a
> WARN_ON_ONCE(acc_mode & MAY_OPENEXEC) for the S_IFSOCK case, since a
> socket cannot be open anyway?
>
We just did some more tests (for the next patch series) and it turns out
that may_open() can return EACCES before another part returns ENXIO.
As a reminder, the next series will deny access to block devices,
character devices, fifo and socket when opened with O_MAYEXEC *and* if
any policy is enforced (via the sysctl).
The question is then: do we prefer to return EACCES when a policy is
enforced (on a socket), or do we stick to the ENXIO? The EACCES approach
will be more consistent with devices and fifo handling, and seems safer
(belt and suspenders) thought.
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