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.
