linux-kernel - Re: [PATCH v6 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O

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Message-ID: <efb88aab-f9f9-4b66-e7ab-3aa054eec96e@digikod.net>
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.