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?
