[PATCH] um: mprotect() __init memory

[Johannes Berg]

From: Johannes Berg <johannes.berg at intel.com>

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.

Signed-off-by: Johannes Berg <johannes.berg at intel.com>
---
 arch/um/kernel/dyn.lds.S |  2 ++
 arch/um/kernel/mem.c     | 11 ++++++++---
 arch/um/kernel/uml.lds.S |  2 ++
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/um/kernel/dyn.lds.S b/arch/um/kernel/dyn.lds.S
index ad3cefeff2ac..5d3d5ef6ebec 100644
--- a/arch/um/kernel/dyn.lds.S
+++ b/arch/um/kernel/dyn.lds.S
@@ -98,8 +98,10 @@ SECTIONS
 
   #include <asm/common.lds.S>
 
+  . = ALIGN(PAGE_SIZE);
   __init_begin = .;
   init.data : { INIT_DATA }
+  . = ALIGN(PAGE_SIZE);
   __init_end = .;
 
   /* Ensure the __preinit_array_start label is properly aligned.  We
diff --git a/arch/um/kernel/mem.c b/arch/um/kernel/mem.c
index 1eef0e42ef5d..00c469fd28ee 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 erroneosly
+ * succeeded - 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);
 }
 
 /* Allocate and free page tables. */
diff --git a/arch/um/kernel/uml.lds.S b/arch/um/kernel/uml.lds.S
index 30aa24348d60..7085ba6fcb93 100644
--- a/arch/um/kernel/uml.lds.S
+++ b/arch/um/kernel/uml.lds.S
@@ -70,8 +70,10 @@ SECTIONS
 
   #include <asm/common.lds.S>
 
+  . = ALIGN(PAGE_SIZE);
   __init_begin = .;
   init.data : { INIT_DATA }
+  . = ALIGN(PAGE_SIZE);
   __init_end = .;
 
   .data    :
-- 
2.55.0