On Wed, Apr 09, 2014 at 06:53:23PM +0100, Al Viro wrote:
> For starters, put that ext4 on top of dm-raid or dm-multipath. That alone
> will very likely push you over the top.
>
> Keep in mind, BTW, that you do not have full 8K to play with - there's
> struct thread_info that should not be stepped upon. Not particulary large
> (IIRC, restart_block is the largest piece in amd64 one), but it eats about
> 100 bytes.
>
> I'd probably use renameat(2) in testing - i.e. trigger the shite when
> resolving a deeply nested symlink in renameat() arguments. That brings
> extra struct nameidata into the game, i.e. extra 152 bytes chewed off the
> stack.
Come to think of that, some extra nastiness could be had by mixing it with
execve(). You can have up to 4 levels of #! resolution there, each eating
up at least 128 bytes (more, actually). Compiler _might_ turn that
tail call of search_binary_handler() into a jump, but it's not guaranteed
at all.
FWIW, it probably makes sense to turn load_script() into
static int load_script(struct linux_binprm *bprm)
{
int err = __load_script(bprm);
if (err)
return err;
return search_binary_handler(bprm);
}
regardless of that issue; we don't need interp[] after the call of
open_exec(), so it makes sense to reduce the footprint in mutual
recursion loop.
For extra pain, consider s/ext4/xfs/, possibly with iscsi thrown under the
bus^Wdm-multipath.
The thing is, we are already too close to stack overflow limit. Adding
several kilobytes more is not survivable, and since you are taking
somebody in a userns DoSing the system into consideration, you can't
say "it takes malicious root to set up, so it's not serious" - the
DoS you mentioned requires the same thing...
BTW, another thing to test would be this:
mount nfs on /mnt
mount a filesystem on /mnt/path that can be invalidated
cd to /mnt/path/foo
bind /mnt on /mnt/path/foo/bar
shoot /mnt/path (on server)
stat bar/path/foo
That should rip the fs you are in out of the tree; it should work, but
it's definitely a case worth testing.
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