From: "Mickaël Salaün" <mic@digikod.net>
To: linux-kernel@vger.kernel.org
Cc: "Mickaël Salaün" <mic@digikod.net>,
"Al Viro" <viro@zeniv.linux.org.uk>,
"James Morris" <jmorris@namei.org>,
"Jonathan Corbet" <corbet@lwn.net>,
"Kees Cook" <keescook@chromium.org>,
"Matthew Garrett" <mjg59@google.com>,
"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>,
"Shuah Khan" <shuah@kernel.org>,
"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: [RFC PATCH v1 5/5] doc: Add documentation for Yama's open_mayexec_enforce
Date: Wed, 12 Dec 2018 09:17:12 +0100 [thread overview]
Message-ID: <20181212081712.32347-6-mic@digikod.net> (raw)
In-Reply-To: <20181212081712.32347-1-mic@digikod.net>
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/LSM/Yama.rst | 41 ++++++++++++++++++++++++++
1 file changed, 41 insertions(+)
diff --git a/Documentation/admin-guide/LSM/Yama.rst b/Documentation/admin-guide/LSM/Yama.rst
index d0a060de3973..a72c86a24b35 100644
--- a/Documentation/admin-guide/LSM/Yama.rst
+++ b/Documentation/admin-guide/LSM/Yama.rst
@@ -72,3 +72,44 @@ The sysctl settings (writable only with ``CAP_SYS_PTRACE``) are:
``PTRACE_TRACEME``. Once set, this sysctl value cannot be changed.
The original children-only logic was based on the restrictions in grsecurity.
+
+open_mayexec_enforce
+====================
+
+The ``O_MAYEXEC`` flag can be passed to :manpage:`open(2)` to only open files
+(or directories) that are 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, Yama enables 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.
+
+Yama implements two complementary security policies to propagate the ``noexec``
+mount option or the executable file permission. These policies are handled by
+the ``kernel.yama.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/yama/test_omayexec.c and
+https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC .
--
2.20.0.rc2
next prev parent reply other threads:[~2018-12-12 8:34 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-12 8:17 [RFC PATCH v1 0/5] Add support for O_MAYEXEC Mickaël Salaün
2018-12-12 8:17 ` [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() Mickaël Salaün
2018-12-12 14:43 ` Jan Kara
2018-12-12 17:09 ` Mickaël Salaün
2018-12-12 20:42 ` Mimi Zohar
2018-12-13 9:47 ` Matthew Bobrowski
2018-12-13 14:23 ` Mickaël Salaün
2019-04-15 18:47 ` Steve Grubb
2019-04-16 11:49 ` Florian Weimer
2019-04-16 15:34 ` Steve Grubb
2019-04-17 10:01 ` Florian Weimer
2019-04-17 15:04 ` Mickaël Salaün
2019-04-17 14:55 ` Mickaël Salaün
2019-08-04 23:55 ` Andy Lutomirski
2019-08-06 16:40 ` Mickaël Salaün
2018-12-12 8:17 ` [RFC PATCH v1 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount propertie Mickaël Salaün
2018-12-12 8:17 ` [RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file executability through O_MAYEXEC Mickaël Salaün
2018-12-12 14:28 ` Mickaël Salaün
2018-12-12 17:09 ` Jann Horn
2018-12-13 14:49 ` Mickaël Salaün
2019-01-03 11:17 ` Jann Horn
2019-01-08 13:29 ` Mickaël Salaün
2019-01-08 23:30 ` Kees Cook
2019-01-09 13:41 ` Mickaël Salaün
2018-12-12 8:17 ` [RFC PATCH v1 4/5] selftest/yama: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2018-12-12 8:17 ` Mickaël Salaün [this message]
2018-12-12 16:29 ` [RFC PATCH v1 0/5] Add support for O_MAYEXEC Jordan Glover
2018-12-12 17:01 ` Mickaël Salaün
2018-12-12 19:51 ` James Morris
2018-12-12 20:13 ` Florian Weimer
2018-12-12 23:40 ` James Morris
2018-12-13 5:13 ` Florian Weimer
2018-12-13 14:57 ` Mickaël Salaün
2018-12-13 3:02 ` Matthew Wilcox
2018-12-13 5:22 ` Florian Weimer
2018-12-13 11:04 ` Mimi Zohar
2018-12-13 11:26 ` Florian Weimer
2018-12-13 12:16 ` Mimi Zohar
2018-12-13 12:16 ` Matthew Wilcox
2018-12-13 15:17 ` Mickaël Salaün
2018-12-13 17:13 ` Matthew Wilcox
2018-12-13 17:36 ` Mickaël Salaün
2018-12-13 17:44 ` Matthew Wilcox
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