Re: [RFC PATCH] mm: extend memfd with ability to create "secret" memory areas

On Thu, 30 Jan 2020 18:23:41 +0200
Mike Rapoport [off-list ref] wrote:

Hi,

This is essentially a resend of my attempt to implement "secret" mappings
using a file descriptor [1]. 

So one little thing I was curious about as I read through the patch...

+static int secretmem_check_limits(struct vm_fault *vmf)
+{
+	struct secretmem_state *state = vmf->vma->vm_file->private_data;
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	unsigned long limit;
+
+	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
+		return -EINVAL;
+
+	limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
+	if (state->nr_pages + 1 >= limit)
+		return -EPERM;
+
+	return 0;
+}

If I'm not mistaken, this means each memfd can be RLIMIT_MEMLOCK in length,
with no global limit on the number of locked pages.  What's keeping me from
creating 1000 of these things and locking down lots of RAM?

Thanks,

jon