On Thu, Nov 02, 2023 at 03:20:51PM -0400, Liam R. Howlett wrote: > * Carlos Llamas <cmllamas@xxxxxxxxxx> [231102 15:00]: > > The mmap read lock is used during the shrinker's callback, which means > > that using alloc->vma pointer isn't safe as it can race with munmap(). > > I think you know my feelings about the safety of that pointer from > previous discussions. > Yeah. The work here is not done. We actually already store the vm_start address in alloc->buffer, so in theory we don't even need to swap the alloc->vma pointer we could just drop it. So, I agree with you. I want to include this saftey "fix" along with some other work that uses the page fault handler and get_user_pages_remote(). I've tried a quick prototype of this and it works fine. > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c > > index e3db8297095a..c4d60d81221b 100644 > > --- a/drivers/android/binder_alloc.c > > +++ b/drivers/android/binder_alloc.c > > @@ -1005,7 +1005,9 @@ enum lru_status binder_alloc_free_page(struct list_head *item, > > goto err_mmget; > > if (!mmap_read_trylock(mm)) > > goto err_mmap_read_lock_failed; > > - vma = binder_alloc_get_vma(alloc); > > + vma = vma_lookup(mm, page_addr); > > + if (vma && vma != binder_alloc_get_vma(alloc)) > > + goto err_invalid_vma; > > Doesn't this need to be: > if (!vma || vma != binder_alloc_get_vma(alloc)) > > This way, we catch a different vma and a NULL vma. > > Or even, just: > if (vma != binder_alloc_get_vma(alloc)) > > if the alloc vma cannot be NULL? > If the vma_lookup() is NULL then we still need to isolate and free the given binder page and we obviously skip the zap() in this case. However, if we receive a random unexpected vma because of a corrupted address or similar, then the whole process is skipped. Thus, why we use the check above. -- Carlos Llamas
