Re: [PATCH 1/2] mmap locking API: Order lock of nascent mm outside lock of live mm

[Jann Horn]

On Fri, Oct 2, 2020 at 11:18 AM Michel Lespinasse <walken@xxxxxxxxxx> wrote:
> On Thu, Oct 1, 2020 at 6:25 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > Until now, the mmap lock of the nascent mm was ordered inside the mmap lock
> > of the old mm (in dup_mmap() and in UML's activate_mm()).
> > A following patch will change the exec path to very broadly lock the
> > nascent mm, but fine-grained locking should still work at the same time for
> > the new mm.
> > To do this in a way that lockdep is happy about, let's turn around the lock
> > ordering in both places that currently nest the locks.
> > Since SINGLE_DEPTH_NESTING is normally used for the inner nesting layer,
> > make up our own lock subclass MMAP_LOCK_SUBCLASS_NASCENT and use that
> > instead.
> >
> > The added locking calls in exec_mmap() are temporary; the following patch
> > will move the locking out of exec_mmap().
>
> Thanks for doing this.
>
> This is probably a silly question, but I am not sure exactly where we
> lock the old MM while bprm is creating the new MM ? I am guessing this
> would be only in setup_arg_pages(), copying the args and environment
> from the old the the new MM ? If that is correct, then wouldn't it be
> sufficient to use mmap_write_lock_nested in setup_arg_pages() ? Or, is
> the issue that we'd prefer to have a killable version of it there ?

We're also implicitly locking the old MM anytime we take page faults
before exec_mmap(), which basically means the various userspace memory
accesses in do_execveat_common(). This happens after bprm_mm_init(),
so we've already set bprm->vma at that point.

> Also FYI I was going to play with these patches a bit to help answer
> these questions on my own, but wasn't able to easily apply them as
> they came lightly mangled (whitespace issues) when I saved them.

Uuugh, dammit, I see what happened. Sorry about the trouble. Thanks
for telling me, guess I'll go back to sending patches the way I did it
before. :/

I guess I'll go make a v2 of this with some extra comment about where
the old MM is accessed, as Jason suggested, and without the whitespace
issues?