[RFC PATCH v1 5/5] doc: Add documentation for Yama's open_mayexec_enforce

From: MickaÃl SalaÃn
Date: Wed Dec 12 2018 - 03:34:12 EST

Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
Reviewed-by: Philippe TrÃbuchet <philippe.trebuchet@xxxxxxxxxxx>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: MickaÃl SalaÃn <mickael.salaun@xxxxxxxxxxx>
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.
+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
+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 .