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

From: Todd Kjos
Date: Mon Jun 27 2022 - 11:44:07 EST


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