linux-kernel - Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys

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Message-ID: <22489759-076e-206a-e367-c13202f05eea@ssi.gouv.fr>
Date:   Thu, 13 Dec 2018 15:23:05 +0100
From:   Mickaël Salaün <mickael.salaun@....gouv.fr>
To:     Matthew Bobrowski <mbobrowski@...browski.org>,
        Jan Kara <jack@...e.cz>
CC:     Mickaël Salaün <mic@...ikod.net>,
        <linux-kernel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>,
        James Morris <jmorris@...ei.org>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>, Shuah Khan <shuah@...nel.org>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>,
        <kernel-hardening@...ts.openwall.com>, <linux-api@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <sgrubb@...hat.com>
Subject: Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()


On 13/12/2018 10:47, Matthew Bobrowski wrote:
> On Wed, Dec 12, 2018 at 03:43:06PM +0100, Jan Kara wrote:
>>> When the O_MAYEXEC flag is passed, sys_open() may be subject to
>>> additional restrictions depending on a security policy implemented by an
>>> LSM through the inode_permission hook.
>>>
>>> The underlying idea is to be able to restrict scripts interpretation
>>> according to a policy defined by the system administrator.  For this to
>>> be possible, script interpreters must use the O_MAYEXEC flag
>>> appropriately.  To be fully effective, these interpreters also need to
>>> handle the other ways to execute code (for which the kernel can't help):
>>> command line parameters (e.g., option -e for Perl), module loading
>>> (e.g., option -m for Python), stdin, file sourcing, environment
>>> variables, configuration files...  According to the threat model, it may
>>> be acceptable to allow some script interpreters (e.g. Bash) to interpret
>>> commands from stdin, may it be a TTY or a pipe, because it may not be
>>> enough to (directly) perform syscalls.
>>>
>>> A simple security policy implementation is available in a following
>>> patch for Yama.
>>>
>>> This is an updated subset of the patch initially written by Vincent
>>> Strubel for CLIP OS:
>>> https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch
>>> This patch has been used for more than 10 years with customized script
>>> interpreters.  Some examples can be found here:
>>> https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
>>>
>>> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
>>> Signed-off-by: Thibaut Sautereau <thibaut.sautereau@....gouv.fr>
>>> Signed-off-by: Vincent Strubel <vincent.strubel@....gouv.fr>
>>> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@....gouv.fr>
>>> Cc: Al Viro <viro@...iv.linux.org.uk>
>>> Cc: Kees Cook <keescook@...omium.org>
>>> Cc: Mickaël Salaün <mickael.salaun@....gouv.fr>
>>
>> ...
>>
>>> diff --git a/fs/open.c b/fs/open.c
>>> index 0285ce7dbd51..75479b79a58f 100644
>>> --- a/fs/open.c
>>> +++ b/fs/open.c
>>> @@ -974,6 +974,10 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
>>>  	if (flags & O_APPEND)
>>>  		acc_mode |= MAY_APPEND;
>>>  
>>> +	/* Check execution permissions on open. */
>>> +	if (flags & O_MAYEXEC)
>>> +		acc_mode |= MAY_OPENEXEC;
>>> +
>>>  	op->acc_mode = acc_mode;
>>>  
>>>  	op->intent = flags & O_PATH ? 0 : LOOKUP_OPEN;
>>
>> I don't feel experienced enough in security to tell whether we want this
>> functionality or not. But if we do this, shouldn't we also set FMODE_EXEC
>> on the resulting struct file? That way also security_file_open() can be
>> used to arbitrate such executable opens and in particular
>> fanotify permission event FAN_OPEN_EXEC will get properly generated which I
>> guess is desirable (support for it is sitting in my tree waiting for the
>> merge window) - adding some audit people involved in FAN_OPEN_EXEC to
>> CC. Just an idea...
> 
> If I'm understanding this patch series correctly, without an enforced LSM
> policy there's realistically no added benefit from a security perspective,
> right?

That's correct. The kernel knows the semantic but the enforcement is
delegated to an LSM and its policy.

> Also, I'm in agreement with what Jan has mentioned in regards to setting
> the __FMODE_EXEC flag when O_MAYEXEC has been specified. This is something that
> would work quite nicely in conjunction with some of the new file access
> notification events.

OK, I will add it in the next patch series (for the new FAN_OPEN_EXEC
support).