On 13/12/2018 10:47, Matthew Bobrowski wrote:

On Wed, Dec 12, 2018 at 03:43:06PM +0100, Jan Kara wrote:

quoted

quoted

When the O_MAYEXEC flag is passed, sys_open() may be subject to
additional restrictions depending on a security policy implemented by an
LSM through the inode_permission hook.

The underlying idea is to be able to restrict scripts interpretation
according to a policy defined by the system administrator.  For this to
be possible, script interpreters must use the O_MAYEXEC flag
appropriately.  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.

A simple security policy implementation is available in a following
patch for Yama.

This is an updated subset of the patch initially written by Vincent
Strubel for CLIP OS:
https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch
This patch has been used for more than 10 years with customized script
interpreters.  Some examples can be found here:
https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Thibaut Sautereau <redacted>
Signed-off-by: Vincent Strubel <redacted>
Reviewed-by: Philippe Trébuchet <redacted>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <redacted>
Cc: Mickaël Salaün <redacted>

...

quoted

diff --git a/fs/open.c b/fs/open.c
index 0285ce7dbd51..75479b79a58f 100644
--- a/fs/open.c
+++ b/fs/open.c

@@ -974,6 +974,10 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
 	if (flags & O_APPEND)
 		acc_mode |= MAY_APPEND;
 
+	/* Check execution permissions on open. */
+	if (flags & O_MAYEXEC)
+		acc_mode |= MAY_OPENEXEC;
+
 	op->acc_mode = acc_mode;
 
 	op->intent = flags & O_PATH ? 0 : LOOKUP_OPEN;

I don't feel experienced enough in security to tell whether we want this
functionality or not. But if we do this, shouldn't we also set FMODE_EXEC
on the resulting struct file? That way also security_file_open() can be
used to arbitrate such executable opens and in particular
fanotify permission event FAN_OPEN_EXEC will get properly generated which I
guess is desirable (support for it is sitting in my tree waiting for the
merge window) - adding some audit people involved in FAN_OPEN_EXEC to
CC. Just an idea...

If I'm understanding this patch series correctly, without an enforced LSM
policy there's realistically no added benefit from a security perspective,
right?

That's correct. The kernel knows the semantic but the enforcement is
delegated to an LSM and its policy.

Also, I'm in agreement with what Jan has mentioned in regards to setting
the __FMODE_EXEC flag when O_MAYEXEC has been specified. This is something that
would work quite nicely in conjunction with some of the new file access
notification events.

OK, I will add it in the next patch series (for the new FAN_OPEN_EXEC
support).