Re: [PATCH 3/3] x86/alternative: make kernel ITS thunks read-only

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 28, 2025 at 02:35:57PM +0200, Juergen Gross wrote:
> When allocating memory pages for kernel ITS thunks, make them read-only
> after having written the last thunk.
> 
> This will be needed when X86_FEATURE_PSE isn't available, as the thunk
> memory will have PAGE_KERNEL_EXEC protection, which is including the
> write permission.
> 
> Cc: <stable@vger.kernel.org>
> Fixes: 5185e7f9f3bd ("x86/module: enable ROX caches for module text on 64 bit")
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/kernel/alternative.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index ecfe7b497cad..bd974a0ac88a 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -217,6 +217,15 @@ static void *its_alloc(void)
>  	return no_free_ptr(page);
>  }
>  
> +static void its_set_kernel_ro(void *addr)
> +{
> +#ifdef CONFIG_MODULES
> +	if (its_mod)
> +		return;
> +#endif
> +	execmem_restore_rox(addr, PAGE_SIZE);
> +}
> +
>  static void *its_allocate_thunk(int reg)
>  {
>  	int size = 3 + (reg / 8);
> @@ -234,6 +243,8 @@ static void *its_allocate_thunk(int reg)
>  #endif
>  
>  	if (!its_page || (its_offset + size - 1) >= PAGE_SIZE) {
> +		if (its_page)
> +			its_set_kernel_ro(its_page);
>  		its_page = its_alloc();
>  		if (!its_page) {
>  			pr_err("ITS page allocation failed\n");
> @@ -2338,6 +2349,11 @@ void __init alternative_instructions(void)
>  	apply_retpolines(__retpoline_sites, __retpoline_sites_end);
>  	apply_returns(__return_sites, __return_sites_end);
>  
> +	/* Make potential last thunk page read-only. */
> +	if (its_page)
> +		its_set_kernel_ro(its_page);
> +	its_page = NULL;
> +
>  	/*
>  	 * Adjust all CALL instructions to point to func()-10, including
>  	 * those in .altinstr_replacement.

No, this is all sorts of wrong. Execmem API should ensure this.