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SubjectRe: [patch V2 1/5] x86/kaiser: Respect disabled CPU features
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Date
> --- a/arch/x86/include/asm/pgtable_64.h
> +++ b/arch/x86/include/asm/pgtable_64.h
> @@ -222,7 +222,8 @@ static inline pgd_t kaiser_set_shadow_pg
> * wrong CR3 value, userspace will crash
> * instead of running.
> */
> - pgd.pgd |= _PAGE_NX;
> + if (__supported_pte_mask & _PAGE_NX)
> + pgd.pgd |= _PAGE_NX;
> }

Thanks for catching that. It's definitely a bug. Although,
practically, it's hard to hit, right? I think everything 64-bit
supports NX unless the hypervisor disabled it or something.

> } else if (pgd_userspace_access(*pgdp)) {
> /*
> --- a/arch/x86/mm/kaiser.c
> +++ b/arch/x86/mm/kaiser.c
> @@ -42,6 +42,8 @@
>
> #define KAISER_WALK_ATOMIC 0x1
>
> +static pteval_t kaiser_pte_mask __ro_after_init = ~(_PAGE_NX | _PAGE_GLOBAL);

Do we need a comment on this, like:

/*
* The NX and GLOBAL bits are not supported on all CPUs.
* We will add them back to this mask at runtime in
* kaiser_init_all_pgds() if supported.
*/

> /*
> * At runtime, the only things we map are some things for CPU
> * hotplug, and stacks for new processes. No two CPUs will ever
> @@ -244,11 +246,14 @@ static pte_t *kaiser_shadow_pagetable_wa
> int kaiser_add_user_map(const void *__start_addr, unsigned long size,
> unsigned long flags)
> {
> - pte_t *pte;
> unsigned long start_addr = (unsigned long)__start_addr;
> unsigned long address = start_addr & PAGE_MASK;
> unsigned long end_addr = PAGE_ALIGN(start_addr + size);
> unsigned long target_address;
> + pte_t *pte;
> +
> + /* Clear not supported bits */
> + flags &= kaiser_pte_mask;

Should we be warning on these if we clear them? Seems kinda funky to
silently fix them up.

> for (; address < end_addr; address += PAGE_SIZE) {
> target_address = get_pa_from_kernel_map(address);
> @@ -308,6 +313,11 @@ static void __init kaiser_init_all_pgds(
> pgd_t *pgd;
> int i;
>
> + if (__supported_pte_mask & _PAGE_NX)
> + kaiser_pte_mask |= _PAGE_NX;
> + if (boot_cpu_has(X86_FEATURE_PGE))
> + kaiser_pte_mask |= _PAGE_GLOBAL;

Practically, I guess boot_cpu_has(X86_FEATURE_PGE) == (cr4_read() &
X86_CR4_PGE). But, in a slow path like this, is it perhaps better to
just be checking CR4 directly?

Looks functionally fine to me, though. Feel free to add my Reviewed-by
or Acked-by.

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