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

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Message-ID: <20250528155821.GD39944@noisy.programming.kicks-ass.net>
Date: Wed, 28 May 2025 17:58:21 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jürgen Groß <jgross@...e.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, xin@...or.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, stable@...r.kernel.org,
	rppt@...nel.org
Subject: Re: [PATCH 3/3] x86/alternative: make kernel ITS thunks read-only

On Wed, May 28, 2025 at 03:30:33PM +0200, Jürgen Groß wrote:

> Have a look at its_fini_mod().

Oh, that's what you mean. But this still isn't very nice, you now have
restore_rox() without make_temp_rw(), which was the intended usage
pattern.

Bah, I hate how execmem works different for !PSE, Mike, you see a sane
way to fix this?

Anyway, if we have to do something like this, then I would prefer it
shaped something like so:

---
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index ecfe7b497cad..33d4d139cb50 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -111,9 +111,8 @@ static bool cfi_paranoid __ro_after_init;
 
 #ifdef CONFIG_MITIGATION_ITS
 
-#ifdef CONFIG_MODULES
 static struct module *its_mod;
-#endif
+static struct its_array its_pages;
 static void *its_page;
 static unsigned int its_offset;
 
@@ -151,68 +150,78 @@ static void *its_init_thunk(void *thunk, int reg)
 	return thunk + offset;
 }
 
-#ifdef CONFIG_MODULES
 void its_init_mod(struct module *mod)
 {
 	if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS))
 		return;
 
-	mutex_lock(&text_mutex);
-	its_mod = mod;
-	its_page = NULL;
+	if (mod) {
+		mutex_lock(&text_mutex);
+		its_mod = mod;
+		its_page = NULL;
+	}
 }
 
 void its_fini_mod(struct module *mod)
 {
+	struct its_array *pages = &its_pages;
+
 	if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS))
 		return;
 
 	WARN_ON_ONCE(its_mod != mod);
 
-	its_mod = NULL;
-	its_page = NULL;
-	mutex_unlock(&text_mutex);
+	if (mod) {
+		pages = &mod->arch.its_pages;
+		its_mod = NULL;
+		its_page = NULL;
+		mutex_unlock(&text_mutex);
+	}
 
-	for (int i = 0; i < mod->its_num_pages; i++) {
-		void *page = mod->its_page_array[i];
+	for (int i = 0; i < pages->num; i++) {
+		void *page = pages->pages[i];
 		execmem_restore_rox(page, PAGE_SIZE);
 	}
+
+	if (!mod)
+		kfree(pages->pages);
 }
 
 void its_free_mod(struct module *mod)
 {
+	struct its_array *pages = &its_pages;
+
 	if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS))
 		return;
 
-	for (int i = 0; i < mod->its_num_pages; i++) {
-		void *page = mod->its_page_array[i];
+	if (mod)
+		pages = &mod->arch.its_pages;
+
+	for (int i = 0; i < pages->num; i++) {
+		void *page = pages->pages[i];
 		execmem_free(page);
 	}
-	kfree(mod->its_page_array);
+	kfree(pages->pages);
 }
-#endif /* CONFIG_MODULES */
 
 static void *its_alloc(void)
 {
-	void *page __free(execmem) = execmem_alloc(EXECMEM_MODULE_TEXT, PAGE_SIZE);
+	struct its_array *pages = &its_pages;
+	void *tmp;
 
+	void *page __free(execmem) = execmem_alloc(EXECMEM_MODULE_TEXT, PAGE_SIZE);
 	if (!page)
 		return NULL;
 
-#ifdef CONFIG_MODULES
-	if (its_mod) {
-		void *tmp = krealloc(its_mod->its_page_array,
-				     (its_mod->its_num_pages+1) * sizeof(void *),
-				     GFP_KERNEL);
-		if (!tmp)
-			return NULL;
+	tmp = krealloc(pages->pages, (pages->num + 1) * sizeof(void *), GFP_KERNEL);
+	if (!tmp)
+		return NULL;
 
-		its_mod->its_page_array = tmp;
-		its_mod->its_page_array[its_mod->its_num_pages++] = page;
+	pages->pages = tmp;
+	pages->pages[pages->num++] = page;
 
+	if (its_mod)
 		execmem_make_temp_rw(page, PAGE_SIZE);
-	}
-#endif /* CONFIG_MODULES */
 
 	return no_free_ptr(page);
 }
@@ -2338,6 +2347,8 @@ void __init alternative_instructions(void)
 	apply_retpolines(__retpoline_sites, __retpoline_sites_end);
 	apply_returns(__return_sites, __return_sites_end);
 
+	its_fini_mod(NULL);
+
 	/*
 	 * Adjust all CALL instructions to point to func()-10, including
 	 * those in .altinstr_replacement.