[PATCH] netfs: Fix netfs_page_mkwrite() to check folio->mapping is valid

[Matthew Wilcox]

On Mon, Jun 24, 2024 at 12:23:01PM +0100, David Howells wrote:
> @@ -508,6 +509,10 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
>  
>  	if (folio_lock_killable(folio) < 0)
>  		goto out;
> +	if (folio->mapping != mapping) {
> +		ret = VM_FAULT_NOPAGE | VM_FAULT_LOCKED;
> +		goto out;
> +	}

Have you tested this?  I'd expect it to throw some VM assertions.

        ret = vmf->vma->vm_ops->page_mkwrite(vmf);
        /* Restore original flags so that caller is not surprised */
        vmf->flags = old_flags;
        if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
                return ret;
...
                if (unlikely(!tmp || (tmp &
                                      (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
                        folio_put(folio);
                        return tmp;
                }

So you locked the folio, then called folio_put() without unlocking it.
Usually the VM complains noisily if you free a locked folio.