Re: [PATCH] binder_alloc: Add missing mmap_lock calls when using the VMA

[Carlos Llamas]

On Tue, Aug 09, 2022 at 04:06:31PM +0000, Liam Howlett wrote:
> Take the mmap_read_lock() when using the VMA in
> binder_alloc_print_pages() and when checking for a VMA in
> binder_alloc_new_buf_locked().
> 
> It is worth noting binder_alloc_new_buf_locked() drops the VMA read lock
> after it verifies a VMA exists, but may be taken again deeper in the
> call stack, if necessary.
> 
> Reported-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> Reported-by: syzbot+a7b60a176ec13cafb793@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: a43cfc87caaf (android: binder: stop saving a pointer to the VMA)
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
>  drivers/android/binder_alloc.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> index f555eebceef6..c23cad7fe313 100644
> --- a/drivers/android/binder_alloc.c
> +++ b/drivers/android/binder_alloc.c
> @@ -395,12 +395,15 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
>  	size_t size, data_offsets_size;
>  	int ret;
>  
> +	mmap_read_lock(alloc->vma_vm_mm);
>  	if (!binder_alloc_get_vma(alloc)) {
> +		mmap_read_unlock(alloc->vma_vm_mm);
>  		binder_alloc_debug(BINDER_DEBUG_USER_ERROR,
>  				   "%d: binder_alloc_buf, no vma\n",
>  				   alloc->pid);
>  		return ERR_PTR(-ESRCH);
>  	}
> +	mmap_read_unlock(alloc->vma_vm_mm);
>  
>  	data_offsets_size = ALIGN(data_size, sizeof(void *)) +
>  		ALIGN(offsets_size, sizeof(void *));
> @@ -922,17 +925,23 @@ void binder_alloc_print_pages(struct seq_file *m,
>  	 * Make sure the binder_alloc is fully initialized, otherwise we might
>  	 * read inconsistent state.
>  	 */
> -	if (binder_alloc_get_vma(alloc) != NULL) {
> -		for (i = 0; i < alloc->buffer_size / PAGE_SIZE; i++) {
> -			page = &alloc->pages[i];
> -			if (!page->page_ptr)
> -				free++;
> -			else if (list_empty(&page->lru))
> -				active++;
> -			else
> -				lru++;
> -		}
> +
> +	mmap_read_lock(alloc->vma_vm_mm);
> +	if (binder_alloc_get_vma(alloc) == NULL)
> +		goto uninitialized;
> +
> +	for (i = 0; i < alloc->buffer_size / PAGE_SIZE; i++) {

do we need to hold on to the lock while we loop through the pages here?

> +		page = &alloc->pages[i];
> +		if (!page->page_ptr)
> +			free++;
> +		else if (list_empty(&page->lru))
> +			active++;
> +		else
> +			lru++;
>  	}
> +
> +uninitialized:
> +	mmap_read_unlock(alloc->vma_vm_mm);
>  	mutex_unlock(&alloc->mutex);
>  	seq_printf(m, "  pages: %d:%d:%d\n", active, lru, free);
>  	seq_printf(m, "  pages high watermark: %zu\n", alloc->pages_high);
> -- 
> 2.35.1