On Thu, Nov 02, 2023 at 03:20:51PM -0400, Liam R. Howlett wrote:
- Carlos Llamas cmllamas@google.com [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