linux-kernel - [PATCH] x86/its: Always return a writable page for dynamic thunks

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Message-ID: <20250604104715.700149-1-nik.borisov@suse.com>
Date: Wed,  4 Jun 2025 13:47:15 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: x86@...nel.org
Cc: pawan.kumar.gupta@...ux.intel.com,
	peterz@...radead.org,
	linux-kernel@...r.kernel.org,
	dave.hansen@...ux.intel.com,
	Nikolay Borisov <nik.borisov@...e.com>
Subject: [PATCH] x86/its: Always return a writable page for dynamic thunks

its_alloc unconditionally allocates and return a ROX page, but setting
it RW is predicated on its_mod being set. This can cause a #GP on the
memset in its_allocate_thunk() if its_mod is not set.

Since the function always returns a page, ensure it's always set
writable so it can be initialized properly.

Fixes: 872df34d7c51 ("x86/its: Use dynamic thunks for indirect branches")
Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>
---
 arch/x86/kernel/alternative.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index ecfe7b497cad..191e909636a5 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -199,6 +199,8 @@ static void *its_alloc(void)
 	if (!page)
 		return NULL;
 
+	execmem_make_temp_rw(page, PAGE_SIZE);
+
 #ifdef CONFIG_MODULES
 	if (its_mod) {
 		void *tmp = krealloc(its_mod->its_page_array,
@@ -210,7 +212,6 @@ static void *its_alloc(void)
 		its_mod->its_page_array = tmp;
 		its_mod->its_page_array[its_mod->its_num_pages++] = page;
 
-		execmem_make_temp_rw(page, PAGE_SIZE);
 	}
 #endif /* CONFIG_MODULES */
 
-- 
2.43.0