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

[Jason Gunthorpe]

On Thu, Oct 01, 2020 at 01:51:33AM +0200, Jann Horn wrote:
> On Thu, Oct 1, 2020 at 1:26 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > On Wed, Sep 30, 2020 at 10:14:57PM +0200, Jann Horn wrote:
> > > 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.
> >
> > Any chance the mmap lock can just be held from mm_struct allocation
> > till exec inserts it into the process?
> 
> Hm... it should work if we define a lockdep subclass for this so that
> lockdep is happy when we call get_user() on the old mm_struct while
> holding that mmap lock.

A subclass isn't right, it has to be a _nested annotation.

nested locking is a pretty good reason to not be able to do this, this
is something lockdep does struggle to model.

Jason