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

[Michel Lespinasse]

On Wed, Sep 30, 2020 at 1:15 PM Jann Horn <jannh@xxxxxxxxxx> 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.
>
> 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?

I'm really not a fan of adding such exceptions; I think it's both
unusual and adds complexity that is not strictly contained into the
init paths.

I don't really understand the concern with the bprm vma in
get_arg_page(); I'm not super familiar with this code but isn't it a
normal vma within the process that __do_execve_file() is creating ? I
received Jason's last email while I was composing this one, but I
think I have the same concern/approach as him, i.e. I think it would
be simplest to keep the new MM locked through the __do_execve_file()
call and avoid adding the mmap_lock_required exception to the
mmap_assert_locked rule.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.