linux-kernel - Re: [PATCH 4/5] x86/its: explicitly manage permissions for ITS pages

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Message-ID: <aD8IeQLZUDvgoQZm@kernel.org>
Date: Tue, 3 Jun 2025 17:36:41 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	J�rgen Gro <jgross@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>, Xin Li <xin@...or.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH 4/5] x86/its: explicitly manage permissions for ITS pages

On Tue, Jun 03, 2025 at 03:58:45PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 03, 2025 at 02:14:44PM +0300, Mike Rapoport wrote:
> > From: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> > 
> > execmem_alloc() sets permissions differently depending on the kernel
> > configuration, CPU support for PSE and whether a page is allocated
> > before or after mark_rodata_ro().
> > 
> > Add tracking for pages allocated for ITS when patching the core kernel
> > and make sure the permissions for ITS pages are explicitly managed for
> > both kernel and module allocations.
> > 
> > Fixes: 872df34d7c51 ("x86/its: Use dynamic thunks for indirect branches")
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Co-developed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > ---
> 
> How about something like this on top?

Works for me :)

> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -121,7 +121,6 @@ struct its_array its_pages;
>  static void *__its_alloc(struct its_array *pages)
>  {
>  	void *page __free(execmem) = execmem_alloc(EXECMEM_MODULE_TEXT, PAGE_SIZE);
> -
>  	if (!page)
>  		return NULL;
>  
> @@ -172,6 +171,9 @@ static void *its_init_thunk(void *thunk,
>  
>  static void its_pages_protect(struct its_array *pages)
>  {
> +	if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
> +		return;
> +

But modules generally use STRICT_MODULE_RWX.
Do you want to make the its pages stricter than normal module text?

>  	for (int i = 0; i < pages->num; i++) {
>  		void *page = pages->pages[i];
>  		execmem_restore_rox(page, PAGE_SIZE);
> @@ -180,8 +182,7 @@ static void its_pages_protect(struct its
>  
>  static void its_fini_core(void)
>  {
> -	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
> -		its_pages_protect(&its_pages);
> +	its_pages_protect(&its_pages);
>  	kfree(its_pages.pages);
>  }
>  
> @@ -207,8 +208,7 @@ void its_fini_mod(struct module *mod)
>  	its_page = NULL;
>  	mutex_unlock(&text_mutex);
>  
> -	if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
> -		its_pages_protect(&mod->arch.its_pages);
> +	its_pages_protect(&mod->arch.its_pages);
>  }

-- 
Sincerely yours,
Mike.