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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>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"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>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
	"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>,
	"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>,
	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
Subject: [PATCH v5 5/6] doc: Add documentation for the fs.open_mayexec_enforce sysctl
Date: Tue,  5 May 2020 17:31:55 +0200
Message-ID: <20200505153156.925111-6-mic@digikod.net> (raw)
In-Reply-To: <20200505153156.925111-1-mic@digikod.net>

This sysctl enables to propagate executable permission to userspace
thanks to the O_MAYEXEC flag.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
---

Changes since v3:
* Switch back to O_MAYEXEC and highlight that it is only taken into
  account by openat2(2).

Changes since v2:
* Update documentation with the new RESOLVE_MAYEXEC.
* Improve explanations, including concerns about LD_PRELOAD.

Changes since v1:
* Move from LSM/Yama to sysctl/fs .
---
 Documentation/admin-guide/sysctl/fs.rst | 44 +++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 2a45119e3331..d55615c36772 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 +166,49 @@ system needs to prune the inode list instead of allocating
 more.
 
 
+open_mayexec_enforce
+--------------------
+
+While being ignored by :manpage:`open(2)` and :manpage:`openat(2)`, the
+``O_MAYEXEC`` flag can be passed to :manpage:`openat2(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,
+or a dynamic linker to only load executable shared objects.  One interesting
+use case is to enforce a "write xor execute" policy through interpreters.
+
+The ability to restrict code execution must be thought as a system-wide policy,
+which first starts by restricting mount points with the ``noexec`` option.
+This option is also automatically applied to special filesystems such as /proc
+.  This prevents files on such mount points to be directly executed by the
+kernel or mapped as executable memory (e.g. libraries).  With script
+interpreters using the ``O_MAYEXEC`` flag, the executable permission can then
+be checked before reading commands from files. This makes it possible to
+enforce the ``noexec`` at the interpreter level, and thus propagates this
+security policy to scripts.  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.
+
+There are two complementary security policies: enforce the ``noexec`` mount
+option, and 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: checks that the mount options for the underlying VFS
+    mount do not prevent execution.
+
+2 - File permission restriction: checks 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/openat2/omayexec_test.c
+and at
+https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC .
+
+
 overflowgid & overflowuid
 -------------------------
 
-- 
2.26.2


  parent reply index

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-05 15:31 [PATCH v5 0/6] Add support for O_MAYEXEC Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 1/6] fs: Add support for an O_MAYEXEC flag on openat2(2) Mickaël Salaün
2020-05-12 21:05   ` Kees Cook
2020-05-12 21:40     ` Christian Heimes
2020-05-12 22:56       ` Kees Cook
2020-05-13 10:13     ` Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property Mickaël Salaün
2020-05-12 21:09   ` Kees Cook
2020-05-14  8:14     ` Lev R. Oshvang .
2020-05-14 15:48       ` Kees Cook
2020-05-17 16:57         ` Lev R. Oshvang .
2020-05-05 15:31 ` [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-05-05 15:44   ` Randy Dunlap
2020-05-05 16:55     ` Mickaël Salaün
2020-05-05 17:40       ` Randy Dunlap
2020-05-12 21:48   ` Kees Cook
2020-05-13 11:09     ` Mickaël Salaün
2020-05-13 15:37   ` Stephen Smalley
2020-05-13 23:27     ` Kees Cook
2020-05-14  3:05       ` Kees Cook
2020-05-14 10:12         ` David Laight
2020-05-14 12:22         ` Stephen Smalley
2020-05-14 14:41           ` Kees Cook
2020-05-14 15:52             ` Stephen Smalley
2020-05-14 15:45           ` Kees Cook
2020-05-14 16:10             ` Stephen Smalley
2020-05-14 19:16               ` Mickaël Salaün
2020-05-15  0:58                 ` Tetsuo Handa
2020-05-15  8:01                 ` How about just O_EXEC? (was Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC) Kees Cook
2020-05-15  8:43                   ` Florian Weimer
2020-05-15 14:37                     ` Kees Cook
2020-05-15 14:43                       ` Florian Weimer
2020-05-15 15:50                         ` Kees Cook
2020-05-18  7:26                           ` Florian Weimer
2020-05-19  2:23                           ` Aleksa Sarai
2020-05-19 10:13                             ` Mickaël Salaün
2020-05-15 11:04                   ` Mickaël Salaün
2020-05-15 15:46                     ` Kees Cook
2020-05-15 18:24                       ` Mickaël Salaün
2020-05-14 19:21       ` [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 4/6] selftest/openat2: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2020-05-12 21:57   ` Kees Cook
2020-05-13 11:18     ` Mickaël Salaün
2020-05-05 15:31 ` Mickaël Salaün [this message]
2020-05-12 22:00   ` [PATCH v5 5/6] doc: Add documentation for the fs.open_mayexec_enforce sysctl Kees Cook
2020-05-13 11:20     ` Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 6/6] ima: add policy support for the new file open MAY_OPENEXEC flag Mickaël Salaün
2020-05-05 15:36 ` [PATCH v5 0/6] Add support for O_MAYEXEC Mickaël Salaün
2020-05-06 13:58   ` Lev R. Oshvang .
2020-05-06 15:41     ` Aleksa Sarai
2020-05-07  8:30     ` Mickaël Salaün
2020-05-07  8:05 ` David Laight
2020-05-07  8:36   ` Mickaël Salaün
2020-05-07  9:00     ` David Laight
2020-05-07  9:30       ` Mickaël Salaün
2020-05-07  9:44         ` David Laight
2020-05-07 13:38           ` Mickaël Salaün
2020-05-08  7:15             ` Lev R. Oshvang .
2020-05-08 14:01               ` Mimi Zohar

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