Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

From: Jan Kara
Date: Wed Dec 12 2018 - 09:43:12 EST

On Wed 12-12-18 09:17:08, Mickaël Salaün wrote:
> 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:
> This patch has been used for more than 10 years with customized script
> interpreters. Some examples can be found here:
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> Signed-off-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx>
> Signed-off-by: Vincent Strubel <vincent.strubel@xxxxxxxxxxx>
> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@xxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Mickaël Salaün <mickael.salaun@xxxxxxxxxxx>


> 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...

Jan Kara <jack@xxxxxxxx>