Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create "secret" memory areas

On Thu, Jul 30, 2020 at 05:22:10PM +0100, Catalin Marinas wrote:

On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:

quoted

+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct secretmem_ctx *ctx = file->private_data;
+	unsigned long mode = ctx->mode;
+	unsigned long len = vma->vm_end - vma->vm_start;
+
+	if (!mode)
+		return -EINVAL;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
+		return -EAGAIN;
+
+	switch (mode) {
+	case SECRETMEM_UNCACHED:
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+		fallthrough;
+	case SECRETMEM_EXCLUSIVE:
+		vma->vm_ops = &secretmem_vm_ops;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	vma->vm_flags |= VM_LOCKED;
+
+	return 0;
+}

I think the uncached mapping is not the right thing for arm/arm64. First
of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
semantics together with not allowing unaligned accesses. I suspect the
semantics are different on x86.

The second, more serious problem, is that I can't find any place where
the caches are flushed for the page mapped on fault. When a page is
allocated, assuming GFP_ZERO, only the caches are guaranteed to be
zeroed. Exposing this subsequently to user space as uncached would allow
the user to read stale data prior to zeroing. The arm64
set_direct_map_default_noflush() doesn't do any cache maintenance.

It's also worth noting that in a virtual machine this is liable to be
either broken (with a potential loss of coherency if the host has a
cacheable alias as existing KVM hosts have), or pointless (if the host
uses S2FWB to upgrade Stage-1 attribues to cacheable as existing KVM
hosts also have).

I think that trying to avoid the data caches creates many more problems
than it solves, and I don't think there's a strong justification for
trying to support that on arm64 to begin with, so I'd rather entirely
opt-out on supporting SECRETMEM_UNCACHED.

Thanks,
Mark.