diff mbox series

[v5,5/6] doc: Add documentation for the fs.open_mayexec_enforce sysctl

Message ID 20200505153156.925111-6-mic@digikod.net
State New, archived
Headers show
Series Add support for O_MAYEXEC | expand

Commit Message

Mickaël Salaün May 5, 2020, 3:31 p.m. UTC
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(+)

Comments

Kees Cook May 12, 2020, 10 p.m. UTC | #1
On Tue, May 05, 2020 at 05:31:55PM +0200, Mickaël Salaün wrote:
> 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>

I think this should be folded into the patch that adds the sysctl.
Mickaël Salaün May 13, 2020, 11:20 a.m. UTC | #2
On 13/05/2020 00:00, Kees Cook wrote:
> On Tue, May 05, 2020 at 05:31:55PM +0200, Mickaël Salaün wrote:
>> 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>
> 
> I think this should be folded into the patch that adds the sysctl.

OK, I guess it is fine for the documentation maintainers too?
diff mbox series

Patch

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
 -------------------------