From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D3ADCC433E6 for ; Thu, 16 Jul 2020 14:19:15 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 0E500207BC for ; Thu, 16 Jul 2020 14:19:14 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0E500207BC Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19349-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 12206 invoked by uid 550); 16 Jul 2020 14:19:07 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 12186 invoked from network); 16 Jul 2020 14:19:06 -0000 Subject: Re: [PATCH v6 4/7] fs: Introduce O_MAYEXEC flag for openat2(2) To: Kees Cook , Jan Kara , Matthew Bobrowski , linux-nfs@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Aleksa Sarai , Alexei Starovoitov , Al Viro , Andrew Morton , Andy Lutomirski , Christian Brauner , Christian Heimes , Daniel Borkmann , Deven Bowers , Dmitry Vyukov , Eric Biggers , Eric Chiang , Florian Weimer , James Morris , Jann Horn , Jonathan Corbet , Lakshmi Ramasubramanian , Matthew Garrett , Matthew Wilcox , Michael Kerrisk , =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= , Mimi Zohar , =?UTF-8?Q?Philippe_Tr=c3=a9buchet?= , Scott Shell , Sean Christopherson , Shuah Khan , Steve Dower , Steve Grubb , Tetsuo Handa , Thibaut Sautereau , Vincent Strubel , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20200714181638.45751-1-mic@digikod.net> <20200714181638.45751-5-mic@digikod.net> <202007151304.9F48071@keescook> From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= Message-ID: Date: Thu, 16 Jul 2020 16:18:27 +0200 User-Agent: MIME-Version: 1.0 In-Reply-To: <202007151304.9F48071@keescook> Content-Type: text/plain; charset=iso-8859-15 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Antivirus: Dr.Web (R) for Unix mail servers drweb plugin ver.6.0.2.8 X-Antivirus-Code: 0x100000 On 15/07/2020 22:06, Kees Cook wrote: > On Tue, Jul 14, 2020 at 08:16:35PM +0200, Mickaël Salaün wrote: >> When the O_MAYEXEC flag is passed, openat2(2) may be subject to >> additional restrictions depending on a security policy managed by the >> kernel through a sysctl or implemented by an LSM thanks to the >> inode_permission hook. This new flag is ignored by open(2) and >> openat(2) because of their unspecified flags handling. >> >> 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: command line parameters (e.g., >> option -e for Perl), module loading (e.g., option -m for Python), stdin, >> file sourcing, environment variables, configuration files, etc. >> 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. Further documentation can be found in a following patch. >> >> Even without enforced security policy, userland interpreters can set it >> to enforce the system policy at their level, knowing that it will not >> break anything on running systems which do not care about this feature. >> However, on systems which want this feature enforced, there will be >> knowledgeable people (i.e. sysadmins who enforced O_MAYEXEC >> deliberately) to manage it. A simple security policy implementation, >> configured through a dedicated sysctl, is available in a following >> patch. >> >> O_MAYEXEC should not be confused with the O_EXEC flag which is intended >> for execute-only, which obviously doesn't work for scripts. However, a >> similar behavior could be implemented in userland with O_PATH: >> https://lore.kernel.org/lkml/1e2f6913-42f2-3578-28ed-567f6a4bdda1@digikod.net/ >> >> The implementation of O_MAYEXEC almost duplicates what execve(2) and >> uselib(2) are already doing: setting MAY_OPENEXEC in acc_mode (which can >> then be checked as MAY_EXEC, if enforced), and propagating FMODE_EXEC to >> _fmode via __FMODE_EXEC flag (which can then trigger a >> fanotify/FAN_OPEN_EXEC event). >> >> This is an updated subset of the patch initially written by Vincent >> Strubel for CLIP OS 4: >> https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch >> This patch has been used for more than 12 years with customized script >> interpreters. Some examples (with the original name O_MAYEXEC) can be >> found here: >> https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC >> >> Co-developed-by: Vincent Strubel >> Signed-off-by: Vincent Strubel >> Co-developed-by: Thibaut Sautereau >> Signed-off-by: Thibaut Sautereau >> Signed-off-by: Mickaël Salaün >> Reviewed-by: Deven Bowers >> Reviewed-by: Kees Cook >> Cc: Aleksa Sarai >> Cc: Al Viro >> --- >> >> Changes since v5: >> * Update commit message. >> >> Changes since v3: >> * Switch back to O_MAYEXEC, but only handle it with openat2(2) which >> checks unknown flags (suggested by Aleksa Sarai). Cf. >> https://lore.kernel.org/lkml/20200430015429.wuob7m5ofdewubui@yavin.dot.cyphar.com/ >> >> Changes since v2: >> * Replace O_MAYEXEC with RESOLVE_MAYEXEC from openat2(2). This change >> enables to not break existing application using bogus O_* flags that >> may be ignored by current kernels by using a new dedicated flag, only >> usable through openat2(2) (suggested by Jeff Layton). Using this flag >> will results in an error if the running kernel does not support it. >> User space needs to manage this case, as with other RESOLVE_* flags. >> The best effort approach to security (for most common distros) will >> simply consists of ignoring such an error and retry without >> RESOLVE_MAYEXEC. However, a fully controlled system may which to >> error out if such an inconsistency is detected. >> >> Changes since v1: >> * Set __FMODE_EXEC when using O_MAYEXEC to make this information >> available through the new fanotify/FAN_OPEN_EXEC event (suggested by >> Jan Kara and Matthew Bobrowski): >> https://lore.kernel.org/lkml/20181213094658.GA996@lithium.mbobrowski.org/ >> --- >> fs/fcntl.c | 2 +- >> fs/open.c | 8 ++++++++ >> include/linux/fcntl.h | 2 +- >> include/linux/fs.h | 2 ++ >> include/uapi/asm-generic/fcntl.h | 7 +++++++ >> 5 files changed, 19 insertions(+), 2 deletions(-) >> >> diff --git a/fs/fcntl.c b/fs/fcntl.c >> index 2e4c0fa2074b..0357ad667563 100644 >> --- a/fs/fcntl.c >> +++ b/fs/fcntl.c >> @@ -1033,7 +1033,7 @@ static int __init fcntl_init(void) >> * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY >> * is defined as O_NONBLOCK on some platforms and not on others. >> */ >> - BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ != >> + BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ != >> HWEIGHT32( >> (VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) | >> __FMODE_EXEC | __FMODE_NONOTIFY)); >> diff --git a/fs/open.c b/fs/open.c >> index 623b7506a6db..38e434bdbbb6 100644 >> --- a/fs/open.c >> +++ b/fs/open.c >> @@ -987,6 +987,8 @@ inline struct open_how build_open_how(int flags, umode_t mode) >> .mode = mode & S_IALLUGO, >> }; >> >> + /* O_MAYEXEC is ignored by syscalls relying on build_open_how(). */ >> + how.flags &= ~O_MAYEXEC; >> /* O_PATH beats everything else. */ >> if (how.flags & O_PATH) >> how.flags &= O_PATH_FLAGS; >> @@ -1054,6 +1056,12 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op) >> if (flags & __O_SYNC) >> flags |= O_DSYNC; >> >> + /* Checks execution permissions on open. */ >> + if (flags & O_MAYEXEC) { >> + acc_mode |= MAY_OPENEXEC; >> + flags |= __FMODE_EXEC; >> + } > > Adding __FMODE_EXEC here will immediately change the behaviors of NFS > and fsnotify. If that's going to happen, I think it needs to be under > the control of the later patches doing the behavioral controls. > (specifically, NFS looks like it completely changes its access control > test when this is set and ignores the read/write checks entirely, which > is not what's wanted). __FMODE_EXEC was suggested by Jan Kara and Matthew Bobrowski because of fsnotify. However, the NFS handling of SUID binaries [1] indeed leads to an unintended behavior. This also means that uselib(2) shouldn't work properly with NFS. I can remove the __FMODE_EXEC flag for now. [1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f8d9a897d4384b77f13781ea813156568f68b83e