On Thu, Dec 13, 2018 at 04:17:29PM +0100, Mickaël Salaün wrote:

On 13/12/2018 04:02, Matthew Wilcox wrote:

quoted

On Wed, Dec 12, 2018 at 09:17:07AM +0100, Mickaël Salaün wrote:

quoted

The goal of this patch series is to control script interpretation.  A
new O_MAYEXEC flag used by sys_open() is added to enable userland script
interpreter to delegate to the kernel (and thus the system security
policy) the permission to interpret scripts or other files containing
what can be seen as commands.

I don't have a problem with the concept, but we're running low on O_ bits.
Does this have to be done before the process gets a file descriptor,
or could we have a new syscall?  Since we're going to be changing the
interpreters anyway, it doesn't seem like too much of an imposition to
ask them to use:

	int verify_for_exec(int fd)

instead of adding an O_MAYEXEC.

Adding a new syscall for this simple use case seems excessive. I think

We have somewhat less than 400 syscalls today.  We have 20 O_ bits defined.
Obviously there's a lower practical limit on syscalls, but in principle
we could have up to 2^32 syscalls, and there are only 12 O_ bits remaining.

that the open/openat syscall familly are the right place to do an atomic
open and permission check, the same way the kernel does for other file
access. Moreover, it will be easier to patch upstream interpreters
without the burden of handling a (new) syscall that may not exist on the
running system, whereas unknown open flags are ignored.

Ah, but that's the problem.  The interpreter can see an -ENOSYS response
and handle it appropriately.  If the flag is silently ignored, the
interpreter has no idea whether it can do a racy check or whether to
skip even trying to do the check.