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: > 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@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... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
