Re: [RFC PATCH v2 17/19] heki: x86: Update permissions counters during text patching

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Nov 12, 2023 at 09:23:24PM -0500, Mickaël Salaün wrote:
> From: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> 
> X86 uses a function called __text_poke() to modify executable code. This
> patching function is used by many features such as KProbes and FTrace.
> 
> Update the permissions counters for the text page so that write
> permissions can be temporarily established in the EPT to modify the
> instructions in that page.
> 
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> Cc: Mickaël Salaün <mic@digikod.net>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Cc: Wanpeng Li <wanpengli@tencent.com>
> Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> ---
> 
> Changes since v1:
> * New patch
> ---
>  arch/x86/kernel/alternative.c |  5 ++++
>  arch/x86/mm/heki.c            | 49 +++++++++++++++++++++++++++++++++++
>  include/linux/heki.h          | 14 ++++++++++
>  3 files changed, 68 insertions(+)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 517ee01503be..64fd8757ba5c 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -18,6 +18,7 @@
>  #include <linux/mmu_context.h>
>  #include <linux/bsearch.h>
>  #include <linux/sync_core.h>
> +#include <linux/heki.h>
>  #include <asm/text-patching.h>
>  #include <asm/alternative.h>
>  #include <asm/sections.h>
> @@ -1801,6 +1802,7 @@ static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t l
>  	 */
>  	pgprot = __pgprot(pgprot_val(PAGE_KERNEL) & ~_PAGE_GLOBAL);
>  
> +	heki_text_poke_start(pages, cross_page_boundary ? 2 : 1, pgprot);
>  	/*
>  	 * The lock is not really needed, but this allows to avoid open-coding.
>  	 */
> @@ -1865,7 +1867,10 @@ static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t l
>  	}
>  
>  	local_irq_restore(flags);
> +
>  	pte_unmap_unlock(ptep, ptl);
> +	heki_text_poke_end(pages, cross_page_boundary ? 2 : 1, pgprot);
> +
>  	return addr;
>  }

This makes no sense, we already use a custom CR3 with userspace alias
for the actual pages to write to, why are you then frobbing permissions
on that *again* ?