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Date: Wed, 12 Dec 2018 18:09:23 +0100
From: Mickaël Salaün <mickael.salaun@....gouv.fr>
To: Jan Kara <jack@...e.cz>, Mickaël Salaün
<mic@...ikod.net>
CC: <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>, Matthew Bobrowski
<mbobrowski@...browski.org>
Subject: Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on
sys_open()
Le 12/12/2018 à 15:43, Jan Kara a écrit :
> On Wed 12-12-18 09:17:08, Mickaël Salaün 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...
Indeed, it may be useful for other LSM.
Mickaël
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