From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() References: <20181212081712.32347-1-mic@digikod.net> <20181212081712.32347-2-mic@digikod.net> <20181212144306.GA19945@quack2.suse.cz> <20181213094658.GA996@lithium.mbobrowski.org> From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= Message-ID: <22489759-076e-206a-e367-c13202f05eea@ssi.gouv.fr> Date: Thu, 13 Dec 2018 15:23:05 +0100 MIME-Version: 1.0 In-Reply-To: <20181213094658.GA996@lithium.mbobrowski.org> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: Matthew Bobrowski , Jan Kara Cc: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= , linux-kernel@vger.kernel.org, Al Viro , James Morris , Jonathan Corbet , Kees Cook , Matthew Garrett , Michael Kerrisk , Mimi Zohar , =?UTF-8?Q?Philippe_Tr=c3=a9buchet?= , Shuah Khan , Thibaut Sautereau , Vincent Strubel , Yves-Alexis Perez , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org, sgrubb@redhat.com List-ID: 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 >>> Signed-off-by: Thibaut Sautereau >>> Signed-off-by: Vincent Strubel >>> Reviewed-by: Philippe Trébuchet >>> Cc: Al Viro >>> Cc: Kees Cook >>> Cc: Mickaël Salaün >> >> ... >> >>> 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).