Re: [PATCH 3/4] mmap locking API: Don't check locking if the mm isn't live yet

[Jann Horn]

On Wed, Sep 30, 2020 at 2:50 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> On Wed, Sep 30, 2020 at 2:30 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > On Tue, Sep 29, 2020 at 06:20:00PM -0700, Jann Horn wrote:
> > > In preparation for adding a mmap_assert_locked() check in
> > > __get_user_pages(), teach the mmap_assert_*locked() helpers that it's fine
> > > to operate on an mm without locking in the middle of execve() as long as
> > > it hasn't been installed on a process yet.
> >
> > I'm happy to see lockdep being added here, but can you elaborate on
> > why add this mmap_locked_required instead of obtaining the lock in the
> > execv path?
>
> My thinking was: At that point, we're logically still in the
> single-owner initialization phase of the mm_struct. Almost any object
> has initialization and teardown steps that occur in a context where
> the object only has a single owner, and therefore no locking is
> required. It seems to me that adding locking in places like
> get_arg_page() would be confusing because it would suggest the
> existence of concurrency where there is no actual concurrency, and it
> might be annoying in terms of lockdep if someone tries to use
> something like get_arg_page() while holding the mmap_sem of the
> calling process. It would also mean that we'd be doing extra locking
> in normal kernel builds that isn't actually logically required.
>
> Hmm, on the other hand, dup_mmap() already locks the child mm (with
> mmap_write_lock_nested()), so I guess it wouldn't be too bad to also
> do it in get_arg_page() and tomoyo_dump_page(), with comments that
> note that we're doing this for lockdep consistency... I guess I can go
> change this in v2.

Actually, I'm taking that back. There's an extra problem:
get_arg_page() accesses bprm->vma, which is set all the way back in
__bprm_mm_init(). We really shouldn't be pretending that we're
properly taking the mmap_sem when actually, we keep reusing a
vm_area_struct pointer.

So for that reason I prefer the approach in the existing patch, where
we make it clear that mm_struct has two different lifetime phases in
which GUP works, and that those lifetime phases have very different
locking requirements.

Does that sound reasonable?