[PATCH 1/9] x86/kexec: Disable global pages before writing to control page

[Kirill A. Shutemov]

On Mon, Dec 16, 2024 at 11:24:08PM +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw at amazon.co.uk>
> 
> The kernel switches to a new set of page tables during kexec. The global
> mappings (_PAGE_GLOBAL==1) can remain in the TLB after this switch. This
> is generally not a problem because the new page tables use a different
> portion of the virtual address space than the normal kernel mappings.
> 
> The critical exception to that generalisation (and the only mapping
> which isn't an identity mapping) is the kexec control page itself —
> which was ROX in the original kernel mapping, but should be RWX in the
> new page tables. If there is a global TLB entry for that in its prior
> read-only state, it definitely needs to be flushed before attempting to
> write through that virtual mapping.
> 
> It would be possible to just avoid writing to the virtual address of the
> page and defer all writes until they can be done through the identity
> mapping. But there's no good reason to keep the old TLB entries around,
> as they can cause nothing but trouble.
> 
> Clear the PGE bit in %cr4 early, before storing data in the control page.

It worth noting that flipping CR4.PGE triggers TLB flush. I was not sure
if CR3 write is required to make it happen.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov