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SubjectRe: bad paravirt/Xen interaction in "x86 - Enhance DEBUG_RODATA support - alternatives"
Jeremy Fitzhardinge wrote:
> The patch "x86 - Enhance DEBUG_RODATA support - alternatives" enables
> the kernel for writing by clearing X86_CR0_WP allow privileged writes.
> This won't work in a paravirt environment for two reasons:
>
> 1. the kernel may not be running in ring 0, so writes will still be
> prevented
> 2. the hypervisor prevents X86_CR0_WP from being cleared anyway (it
> GPFs the cr0 update)
>
> This crashes on Xen, and it would probably break VMI too.
>
> The only safe way to allow writes is to change the page permissions
> (either on the page itself, or create a temporary writable alias for
> that page). Perhaps something you could do it with kmap_atomic.
>

A properly implemented hypervisor should arguably emulate this.

Doesn't really mean the patch is worth the pain.

-hpa


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