From a418ab8fddea49f937f50ac860314ce5d3d9398a Mon Sep 17 00:00:00 2001
From: Johannes Berg <johannes.berg@intel.com>
Date: Thu, 24 Sep 2026 08:49:45 +0200
Subject: [PATCH] um: mprotect() __init memory

Unlike what the comment says, we could munmap() this (but
not reuse it for guest allocations), but then stray libc
allocations could technically conflict, so mprotect() it
to catch access bugs. Align it in the linker scripts too
so that all of it can be covered, not just some.

Link: https://patch.msgid.link/20260924084946.d118ec78e604.I993b671566a4e842dcc290b78f1c0004050362ba@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
 arch/um/kernel/dyn.lds.S |  3 +--
 arch/um/kernel/mem.c     | 11 ++++++++---
 arch/um/kernel/uml.lds.S |  1 +
 3 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/um/kernel/dyn.lds.S b/arch/um/kernel/dyn.lds.S
index 32b60272a4fa..31adf9451507 100644
--- a/arch/um/kernel/dyn.lds.S
+++ b/arch/um/kernel/dyn.lds.S
@@ -24,9 +24,8 @@ SECTIONS
   INIT_TEXT_SECTION(PAGE_SIZE)
 
   init.data : { INIT_DATA }
-  __init_end = .;
-
   . = ALIGN(PAGE_SIZE);
+  __init_end = .;
 
   /* Read-only sections, merged into text segment: */
   .hash           : { *(.hash) }
diff --git a/arch/um/kernel/mem.c b/arch/um/kernel/mem.c
index bc03bf3d114d..2219827ac3ad 100644
--- a/arch/um/kernel/mem.c
+++ b/arch/um/kernel/mem.c
@@ -83,12 +83,17 @@ void __init arch_zone_limits_init(unsigned long *max_zone_pfns)
 }
 
 /*
- * This can't do anything because nothing in the kernel image can be freed
- * since it's not in kernel physical memory.
+ * We could munmap() this instead, but then libc allocations could
+ * land in this area and stray initdata access could erroneously
+ * succeed - just mprotect() it to reliably catch bad accesses.
  */
-
 void free_initmem(void)
 {
+	unsigned long start = PAGE_ALIGN((unsigned long)__init_begin);
+	unsigned long end = round_down((unsigned long)__init_end, PAGE_SIZE);
+
+	if (end > start)
+		os_protect_memory((void *)start, end - start, 0, 0, 0);
 }
 
 void *uml_kmalloc(int size, int flags)
diff --git a/arch/um/kernel/uml.lds.S b/arch/um/kernel/uml.lds.S
index 7a6de1c74d6d..4aabcb7faa38 100644
--- a/arch/um/kernel/uml.lds.S
+++ b/arch/um/kernel/uml.lds.S
@@ -32,6 +32,7 @@ SECTIONS
   INIT_TEXT_SECTION(0)
 
   init.data : { INIT_DATA }
+  . = ALIGN(PAGE_SIZE);
   __init_end = .;
 
   .text      :