Re: [PATCH 4.19 84/86] efi/x86: Handle by-ref arguments covering multiple pages in mixed mode

[Greg Kroah-Hartman]

On Wed, Mar 11, 2020 at 02:01:07PM +0100, Pavel Machek wrote:
> Hi!
> 
> > Currently, the mixed mode runtime service wrappers require that all by-ref
> > arguments that live in the vmalloc space have a size that is a power of 2,
> > and are aligned to that same value. While this is a sensible way to
> > construct an object that is guaranteed not to cross a page boundary, it is
> > overly strict when it comes to checking whether a given object violates
> > this requirement, as we can simply take the physical address of the first
> > and the last byte, and verify that they point into the same physical
> > page.
> 
> Dunno. If start passing buffers that _sometime_ cross page boundaries,
> we'll get hard to debug failures. Maybe original code is better
> buecause it catches problems earlier?
> 
> Furthermore, all existing code should pass aligned, 2^n size buffers,
> so we should not need this in stable?

For some crazy reason you cut out the reason I applied this patch to the
stable tree.  From the changelog text:
	Fixes: f6697df36bdf0bf7 ("x86/efi: Prevent mixed mode boot corruption with CONFIG_VMAP_STACK=y")