Re: [PATCH] x86/boot: Fix PVH boot during boot_info transition period

[Alan.Robinson@xxxxxxxxxxx (Alan Robinson)]

Hi Andrew,

A small suggestion for the commit log..

On Tue, Oct 22, 2024 at 12:41:14PM +0000, Andrew Cooper wrote:
> 
> multiboot_fill_boot_info() taking the physical address of the multiboot_info
> structure leads to a subtle use-after-free on the PVH path, with rather less
> subtle fallout.
> 
> The pointers used by __start_xen(), mbi and mod, are either:
> 
>  - MB:  Directmap pointers into the trampoline, or
>  - PVH: Xen pointers into .initdata, or
>  - EFI: Directmap pointers into Xen.
> 
> Critically, these either remain valid across move_xen() (MB, PVH), or rely on
> move_xen() being inhibited (EFI).
> 
> The conversion to multiboot_fill_boot_info(), taking only mbi_p, makes the PVH
> path use directmap pointers into Xen, as well as move_xen() which invalidates
> said pointers.
> 
> Switch multiboot_fill_boot_info() to consume the same virtual addresses that
> __start_xen() currently uses.  This keeps all the pointers valid for the
> duration of __start_xen(), for all boot protocols.
> 
> It can be safely untangled once multiboot_fill_boot_info() takes a full copy
> the multiboot info data, and __start_xen() has been moved over to using the

 of the multiboot info data, and __start_xen() has been moved over to using the

> new boot_info consistently.
> 

Alan