[adding more people because this is going to be an ABI break, sigh]
On Sat, Oct 12, 2019 at 5:52 PM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>
> On Sat, Oct 12, 2019 at 4:10 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > On Sat, Oct 12, 2019 at 12:16 PM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> > >
> > > The new secure flag makes userfaultfd use a new "secure" anonymous
> > > file object instead of the default one, letting security modules
> > > supervise userfaultfd use.
> > >
> > > Requiring that users pass a new flag lets us avoid changing the
> > > semantics for existing callers.
> >
> > Is there any good reason not to make this be the default?
> >
> >
> > The only downside I can see is that it would increase the memory usage
> > of userfaultfd(), but that doesn't seem like such a big deal. A
> > lighter-weight alternative would be to have a single inode shared by
> > all userfaultfd instances, which would require a somewhat different
> > internal anon_inode API.
>
> I'd also prefer to just make SELinux use mandatory, but there's a
> nasty interaction with UFFD_EVENT_FORK. Adding a new UFFD_SECURE mode
> which blocks UFFD_EVENT_FORK sidesteps this problem. Maybe you know a
> better way to deal with it.
...
> But maybe we can go further: let's separate authentication and
> authorization, as we do in other LSM hooks. Let's split my
> inode_init_security_anon into two hooks, inode_init_security_anon and
> inode_create_anon. We'd define the former to just initialize the file
> object's security information --- in the SELinux case, figuring out
> its class and SID --- and define the latter to answer the yes/no
> question of whether a particular anonymous inode creation should be
> allowed. Normally, anon_inode_getfile2() would just call both hooks.
> We'd add another anon_inode_getfd flag, ANON_INODE_SKIP_AUTHORIZATION
> or something, that would tell anon_inode_getfile2() to skip calling
> the authorization hook, effectively making the creation always
> succeed. We can then make the UFFD code pass
> ANON_INODE_SKIP_AUTHORIZATION when it's creating a file object in the
> fork child while creating UFFD_EVENT_FORK messages.
That sounds like an improvement. Or maybe just teach SELinux that
this particular fd creation is actually making an anon_inode that is a
child of an existing anon inode and that the context should be copied
or whatever SELinux wants to do. Like this, maybe:
static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
struct userfaultfd_ctx *new,
struct uffd_msg *msg)
{
int fd;
Change this:
fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
to something like:
fd = anon_inode_make_child_fd(..., ctx->inode, ...);
where ctx->inode is the one context's inode.
*** HOWEVER *** !!!
Now that you've pointed this mechanism out, it is utterly and
completely broken and should be removed from the kernel outright or at
least severely restricted. A .read implementation MUST NOT ACT ON THE
CALLING TASK. Ever. Just imagine the effect of passing a userfaultfd
as stdin to a setuid program.
So I think the right solution might be to attempt to *remove*
UFFD_EVENT_FORK. Maybe the solution is to say that, unless the
creator of a userfaultfd() has global CAP_SYS_ADMIN, then it cannot
use UFFD_FEATURE_EVENT_FORK) and print a warning (once) when
UFFD_FEATURE_EVENT_FORK is allowed. And, after some suitable
deprecation period, just remove it. If it's genuinely useful, it
needs an entirely new API based on ioctl() or a syscall. Or even
recvmsg() :)
And UFFD_SECURE should just become automatic, since you don't have a
problem any more. :-p
--Andy
