linux-kernel - [PATCH v3 5/5] doc: Add documentation for the fs.open_mayexec

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Date:   Tue, 28 Apr 2020 19:51:29 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     linux-kernel@...r.kernel.org
Cc:     Mickaël Salaün <mic@...ikod.net>,
        Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Eric Chiang <ericchiang@...gle.com>,
        Florian Weimer <fweimer@...hat.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Scott Shell <scottsh@...rosoft.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: [PATCH v3 5/5] doc: Add documentation for the fs.open_mayexec_enforce sysctl

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

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 | 43 +++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 2a45119e3331..1ed65a38d5fb 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 ``RESOLVE_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.  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 ``RESOLVE_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 (with the original ``O_MAYEXEC``) at
+https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC .
+
+
 overflowgid & overflowuid
 -------------------------
 
-- 
2.26.2