[PATCH 4/5] x86/its: explicitly manage permissions for ITS pages

[Mike Rapoport]

From: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Co-developed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
---
 arch/x86/kernel/alternative.c | 84 ++++++++++++++++++++++++++---------
 1 file changed, 63 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 372ef5dff631..8289e9e1f954 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -138,6 +138,25 @@ static struct module *its_mod;
 #endif
 static void *its_page;
 static unsigned int its_offset;
+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;
+
+	void *tmp = krealloc(pages->pages, (pages->num+1) * sizeof(void *),
+			     GFP_KERNEL);
+	if (!tmp)
+		return NULL;
+
+	pages->pages = tmp;
+	pages->pages[pages->num++] = page;
+
+	return no_free_ptr(page);
+}
 
 /* Initialize a thunk with the "jmp *reg; int3" instructions. */
 static void *its_init_thunk(void *thunk, int reg)
@@ -173,6 +192,21 @@ static void *its_init_thunk(void *thunk, int reg)
 	return thunk + offset;
 }
 
+static void its_pages_protect(struct its_array *pages)
+{
+	for (int i = 0; i < pages->num; i++) {
+		void *page = pages->pages[i];
+		execmem_restore_rox(page, PAGE_SIZE);
+	}
+}
+
+static void its_fini_core(void)
+{
+	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
+		its_pages_protect(&its_pages);
+	kfree(its_pages.pages);
+}
+
 #ifdef CONFIG_MODULES
 void its_init_mod(struct module *mod)
 {
@@ -195,10 +229,8 @@ void its_fini_mod(struct module *mod)
 	its_page = NULL;
 	mutex_unlock(&text_mutex);
 
-	for (int i = 0; i < mod->arch.its_pages.num; i++) {
-		void *page = mod->arch.its_pages.pages[i];
-		execmem_restore_rox(page, PAGE_SIZE);
-	}
+	if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
+		its_pages_protect(&mod->arch.its_pages);
 }
 
 void its_free_mod(struct module *mod)
@@ -212,32 +244,38 @@ void its_free_mod(struct module *mod)
 	}
 	kfree(mod->arch.its_pages.pages);
 }
-#endif /* CONFIG_MODULES */
 
-static void *its_alloc(void)
+static void *its_alloc_mod(void)
 {
-	void *page __free(execmem) = execmem_alloc(EXECMEM_MODULE_TEXT, PAGE_SIZE);
+	void *page = __its_alloc(&its_mod->arch.its_pages);
 
-	if (!page)
-		return NULL;
+	if (page)
+		execmem_make_temp_rw(page, PAGE_SIZE);
 
-#ifdef CONFIG_MODULES
-	if (its_mod) {
-		struct its_array *pages = &its_mod->arch.its_pages;
-		void *tmp = krealloc(pages->pages,
-				     (pages->num+1) * sizeof(void *),
-				     GFP_KERNEL);
-		if (!tmp)
-			return NULL;
+	return page;
+}
+#endif /* CONFIG_MODULES */
 
-		pages->pages = tmp;
-		pages->pages[pages->num++] = page;
+static void *its_alloc_core(void)
+{
+	void *page = __its_alloc(&its_pages);
 
+	if (page) {
 		execmem_make_temp_rw(page, PAGE_SIZE);
+		set_memory_x((unsigned long)page, 1);
 	}
+
+	return page;
+}
+
+static void *its_alloc(void)
+{
+#ifdef CONFIG_MODULES
+	if (its_mod)
+		return its_alloc_mod();
 #endif /* CONFIG_MODULES */
 
-	return no_free_ptr(page);
+	return its_alloc_core();
 }
 
 static void *its_allocate_thunk(int reg)
@@ -291,7 +329,9 @@ u8 *its_static_thunk(int reg)
 	return thunk;
 }
 
-#endif
+#else
+static inline void its_fini_core(void) {}
+#endif /* CONFIG_MITIGATION_ITS */
 
 /*
  * Nomenclature for variable names to simplify and clarify this code and ease
@@ -2368,6 +2408,8 @@ void __init alternative_instructions(void)
 	apply_retpolines(__retpoline_sites, __retpoline_sites_end);
 	apply_returns(__return_sites, __return_sites_end);
 
+	its_fini_core();
+
 	/*
 	 * Adjust all CALL instructions to point to func()-10, including
 	 * those in .altinstr_replacement.
-- 
2.47.2