Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

On 11/25/21 2:05 AM, Joerg Roedel wrote:

On Wed, Nov 24, 2021 at 09:48:14AM -0800, Dave Hansen wrote:

quoted

That covers things like copy_from_user().  It does not account for
things where kernel mappings are used, like where a
get_user_pages()/kmap() is in play.

The kmap case is guarded by KVM code, which locks the page first so that
the guest can't change the page state, then checks the page state, and
if it is shared does the kmap and the access.

This should turn an RMP fault in the kernel which is not covered in the
uaccess exception table into a fatal error.

Let's say something does process_vm_readv() where the pid is a qemu
process and it is writing to a guest private memory area.  The syscall
will eventually end up in process_vm_rw_single_vec() which does:

                pinned_pages = pin_user_pages_remote(mm, pa, pinned_pages,
                                                     flags, process_pages,
                                                     NULL, &locked);

...

                rc = process_vm_rw_pages(process_pages,
                                         start_offset, bytes, iter,
                                         vm_write);

and eventually in copy_page_from_iter():

                void *kaddr = kmap_local_page(page);
                size_t wanted = _copy_from_iter(kaddr + offset, bytes, i);
                kunmap_local(kaddr);

The kernel access to 'kaddr+offset' shouldn't fault.  How does the KVM
code thwart that kmap_local_page()?