Le 12/12/2018 à 15:43, Jan Kara a écrit : > 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... Indeed, it may be useful for other LSM. Mickaël
