Re: netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Nov 30, 2017 at 04:57:30PM -0800, Kees Cook wrote:
> On Mon, Oct 9, 2017 at 4:10 PM, David Miller <davem@davemloft.net> wrote:
> > Shmulik Ladkani (1):
> >       netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'
> 
> This adds a new user of set_fs(), which we're trying to eliminate (or
> at least not expand):
> 
> +       set_fs(KERNEL_DS);
> +       fd = bpf_obj_get_user(path);
> +       set_fs(oldfs);
> 
> Can you please adjust this to not make set_fs() changes?

That's not the worst problem there.  Messing with descriptor table is much
worse.  It can be shared between threads; by the time you get to fdget()
the damn thing might have nothing to do with what bpf_obj_get_user() has
put there, ditto for sys_close().

Use of file descriptors should be limited to "got a number from userland,
convert to struct file *" on the way in and "install struct file * into
descriptor table and return the descriptor to userland" on the way out.
And the latter - *ONLY* after the last possible point of failure.  Once
a file reference is inserted into descriptor table, that's it - you
can't undo that.

The only way to use bpf_obj_get_user() is to pass its return value to
userland.  As return value of syscall - not even put_user() (for that
you'd need to reserve the descriptor, copy it to userland and only
then attach struct file * to it).

The whole approach stinks - what it needs is something that would
take struct filename * and return struct bpf_prog * or struct file *
reference.  With bpf_obj_get_user() and this thing implemented
via that.

I'm looking into that thing...