Re: Simplify load_unaligned_zeropad() (was Re: [GIT PULL] Ceph updates for 5.20-rc1)

[Dave Hansen]

On 8/14/22 21:12, Kirill A. Shutemov wrote:
> But if somebody would try to be clever -- allocate memory and vmap() as
> shared (with proper VMM notification), but leave direct mapping intact --
> we have a problem. load_unaligned_zeropad() can step onto private mapping
> of the shared memory in direct mapping and crash whole TD guest.
> 
> The worst part is that for somebody who is not aware about
> load_unaligned_zeropad(), the vmap() trick is totally reasonable approach:
> it helps to avoid direct mapping fragmentation. We considered the trick
> for one of TDX-specific drivers.

This can also be fixed with guard pages.  It means allocating a
physically contiguous page to precede each one you want to vmap(), but
that's not the end of the world.

We're going to have to be *VERY* careful with any private->shared
conversion code.