Date: Wed, 12 Dec 2018 09:17:12 +0100 From: Mickaël Salaün <mic@...ikod.net> To: linux-kernel@...r.kernel.org Cc: Mickaël Salaün <mic@...ikod.net>, Al Viro <viro@...iv.linux.org.uk>, James Morris <jmorris@...ei.org>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Matthew Garrett <mjg59@...gle.com>, Michael Kerrisk <mtk.manpages@...il.com>, Mickaël Salaün <mickael.salaun@....gouv.fr>, Mimi Zohar <zohar@...ux.ibm.com>, Philippe Trébuchet <philippe.trebuchet@....gouv.fr>, Shuah Khan <shuah@...nel.org>, Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>, Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>, kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org, linux-security-module@...r.kernel.org, linux-fsdevel@...r.kernel.org Subject: [RFC PATCH v1 5/5] doc: Add documentation for Yama's open_mayexec_enforce Signed-off-by: Mickaël Salaün <mic@...ikod.net> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@....gouv.fr> Reviewed-by: Thibaut Sautereau <thibaut.sautereau@....gouv.fr> Cc: Jonathan Corbet <corbet@....net> Cc: Kees Cook <keescook@...omium.org> Cc: Mickaël Salaün <mickael.salaun@....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
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.