Re: [PATCH] android: binder: Fix lockdep check on clearing vma

[Todd Kjos]

On Mon, Jun 27, 2022 at 8:19 AM Liam Howlett <liam.howlett@xxxxxxxxxx> wrote:
>
> When munmapping a vma, the mmap_lock can be degraded to a write before
> calling close() on the file handle.  The binder close() function calls
> binder_alloc_set_vma() to clear the vma address, which now has a lock
> dep check for writing on the mmap_lock.  Change the lockdep check to
> ensure the reading lock is held while clearing and keep the write check
> while writing.
>
> Reported-by: syzbot+da54fa8d793ca89c741f@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 472a68df605b ("android: binder: stop saving a pointer to the VMA")
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

Acked-by: Todd Kjos <tkjos@xxxxxxxxxx>
> ---
>  drivers/android/binder_alloc.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> index f555eebceef6..1014beb12802 100644
> --- a/drivers/android/binder_alloc.c
> +++ b/drivers/android/binder_alloc.c
> @@ -315,12 +315,19 @@ static inline void binder_alloc_set_vma(struct binder_alloc *alloc,
>  {
>         unsigned long vm_start = 0;
>
> +       /*
> +        * Allow clearing the vma with holding just the read lock to allow
> +        * munmapping downgrade of the write lock before freeing and closing the
> +        * file using binder_alloc_vma_close().
> +        */
>         if (vma) {
>                 vm_start = vma->vm_start;
>                 alloc->vma_vm_mm = vma->vm_mm;
> +               mmap_assert_write_locked(alloc->vma_vm_mm);
> +       } else {
> +               mmap_assert_locked(alloc->vma_vm_mm);
>         }
>
> -       mmap_assert_write_locked(alloc->vma_vm_mm);
>         alloc->vma_addr = vm_start;
>  }
>
> --
> 2.35.1