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Message-Id: <20200428175129.634352-6-mic@digikod.net>
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
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