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From: "Mickaël Salaün" <mic@digikod.net>
To: linux-kernel@vger.kernel.org
Cc: "Mickaël Salaün" <mic@digikod.net>,
	"Aleksa Sarai" <cyphar@cyphar.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Eric Chiang" <ericchiang@google.com>,
	"Florian Weimer" <fweimer@redhat.com>,
	"James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Kees Cook" <keescook@chromium.org>,
	"Matthew Garrett" <mjg59@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michael Kerrisk" <mtk.manpages@gmail.com>,
	"Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	"Philippe Trébuchet" <philippe.trebuchet@ssi.gouv.fr>,
	"Scott Shell" <scottsh@microsoft.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Song Liu" <songliubraving@fb.com>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>,
	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	"Yves-Alexis Perez" <yves-alexis.perez@ssi.gouv.fr>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: [PATCH v2 5/5] doc: Add documentation for the fs.open_mayexec_enforce sysctl
Date: Fri,  6 Sep 2019 17:24:55 +0200
Message-ID: <20190906152455.22757-6-mic@digikod.net> (raw)
In-Reply-To: <20190906152455.22757-1-mic@digikod.net>

Changes since v1:
* move from LSM/Yama to sysctl/fs

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mickaël Salaün <mickael.salaun@ssi.gouv.fr>
---
 Documentation/admin-guide/sysctl/fs.rst | 43 +++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 2a45119e3331..f2f5bbe428d6 100644
--- a/Documentation/admin-guide/sysctl/fs.rst
+++ b/Documentation/admin-guide/sysctl/fs.rst
@@ -37,6 +37,7 @@ Currently, these files are in /proc/sys/fs:
 - inode-nr
 - inode-state
 - nr_open
+- open_mayexec_enforce
 - overflowuid
 - overflowgid
 - pipe-user-pages-hard
@@ -165,6 +166,48 @@ system needs to prune the inode list instead of allocating
 more.
 
 
+open_mayexec_enforce
+--------------------
+
+The ``O_MAYEXEC`` flag can be passed to :manpage:`open(2)` to only open regular
+files that are expected to be executable.  If the file is not identified as
+executable, then the syscall returns -EACCES.  This may allow a script
+interpreter to check executable permission before reading commands from a file.
+One interesting use case is to enforce a "write xor execute" policy through
+interpreters.
+
+Thanks to this flag, it is possible to enforce the ``noexec`` mount option
+(i.e.  the underlying mount point of the file is mounted with MNT_NOEXEC or its
+underlying superblock is SB_I_NOEXEC) not only on ELF binaries but also on
+scripts.  This may be possible thanks to script interpreters using the
+``O_MAYEXEC`` flag.  The executable permission is then checked before reading
+commands from a file, and thus can enforce the ``noexec`` at the interpreter
+level by propagating this security policy to the scripts.  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.
+
+There is two complementary security policies: enforce the ``noexec`` mount
+option, or enforce executable file permission.  These policies are handled by
+the ``fs.open_mayexec_enforce`` sysctl (writable only with ``CAP_MAC_ADMIN``)
+as a bitmask:
+
+1 - mount restriction:
+    check that the mount options for the underlying VFS mount do not prevent
+    execution.
+
+2 - file permission restriction:
+    check that the to-be-opened file is marked as executable for the current
+    process (e.g., POSIX permissions).
+
+Code samples can be found in tools/testing/selftests/exec/omayexec.c and
+https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC .
+
+
 overflowgid & overflowuid
 -------------------------
 
-- 
2.23.0


  parent reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-06 15:24 [PATCH v2 0/5] Add support for O_MAYEXEC Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() Mickaël Salaün
2019-09-06 15:56   ` Florian Weimer
2019-09-06 16:06     ` Mickaël Salaün
2019-09-06 16:48       ` Jeff Layton
2019-09-06 17:13         ` Aleksa Sarai
2019-09-06 19:43           ` Jeff Layton
2019-09-06 20:06             ` Andy Lutomirski
2019-09-06 20:51               ` Jeff Layton
2019-09-06 21:27                 ` Andy Lutomirski
2019-09-06 22:12                 ` Aleksa Sarai
2019-09-09  9:33               ` Mickaël Salaün
2019-09-06 22:05             ` Aleksa Sarai
2019-09-06 22:18               ` Aleksa Sarai
2019-09-06 17:14         ` Mickaël Salaün
2019-09-06 18:38           ` Jeff Layton
2019-09-06 18:41             ` Andy Lutomirski
2019-09-09  9:18               ` Mickaël Salaün
2019-09-09 15:49                 ` Andy Lutomirski
2019-09-06 18:44             ` Florian Weimer
2019-09-06 19:03             ` James Morris
2019-09-09  9:25               ` Mickaël Salaün
2019-09-09 10:12                 ` James Morris
2019-09-09 10:54                   ` Mickaël Salaün
2019-09-09 12:28                     ` Aleksa Sarai
2019-09-09 12:33                       ` Mickaël Salaün
2019-09-09 11:54                 ` Aleksa Sarai
2019-09-09 12:28                   ` Mickaël Salaün
2019-09-06 17:07       ` Aleksa Sarai
2019-09-06 17:20         ` Christian Brauner
2019-09-06 17:24           ` Mickaël Salaün
2019-09-06 17:40           ` Tycho Andersen
2019-09-06 18:27             ` Florian Weimer
2019-09-06 18:46               ` Tycho Andersen
2019-09-06 15:24 ` [PATCH v2 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount propertie Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 3/5] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 4/5] selftest/exec: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2019-09-06 15:24 ` Mickaël Salaün [this message]
2019-09-06 18:50 ` [PATCH v2 0/5] Add support for O_MAYEXEC Steve Grubb
2019-09-06 18:57   ` Florian Weimer
2019-09-06 19:07     ` Steve Grubb
2019-09-06 19:26       ` Andy Lutomirski
2019-09-06 22:44         ` Aleksa Sarai
2019-09-09  9:09           ` Mickaël Salaün
2019-09-09  0:16 ` James Morris

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