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

[Juergen Gross <jgross@suse.com>]

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.
-- 
2.43.0