[PATCH v2 2/3] module: Make srcu_struct ptr array as read-only post init

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

Since commit title ("srcu: Allocate per-CPU data for DEFINE_SRCU() in
modules"), modules that call DEFINE_{STATIC,}SRCU will have a new array
of srcu_struct pointers which is used by srcu code to initialize and
clean up these structures.

There is no reason for this array of pointers to be writable, and can
cause security or other hidden bugs. Mark these are read-only after the
module init has completed.

Suggested-by: paulmck@linux.vnet.ibm.com
Suggested-by: keescook@chromium.org
Acked-by: keescook@chromium.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/module.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 1acddb93282a..8b9631e789f0 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3305,7 +3305,7 @@ core_param(module_blacklist, module_blacklist, charp, 0400);
  * layout_sections() can put it in the right place.
  * Note: ro_after_init sections also have SHF_{WRITE,ALLOC} set.
  */
-static char *ro_after_init_sections[] = {
+static const char * const ro_after_init_sections[] = {
 	".data..ro_after_init",
 
 	/*
@@ -3314,6 +3314,12 @@ static char *ro_after_init_sections[] = {
 	 * annotated as such at module load time.
 	 */
 	"__jump_table",
+
+	/*
+	 * Used for SRCU structures which need to be initialized/cleaned up
+	 * by the SRCU notifiers
+	 */
+	"___srcu_struct_ptrs",
 };
 
 static struct module *layout_and_allocate(struct load_info *info, int flags)
@@ -3336,7 +3342,7 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 	info->sechdrs[info->index.pcpu].sh_flags &= ~(unsigned long)SHF_ALLOC;
 
 	/* Set sh_flags for read-only after init sections */
-	for (i = 0; ro_after_init_sections[i]; i++) {
+	for (i = 0; i < ARRAY_SIZE(ro_after_init_sections); i++) {
 		ndx = find_sec(info, ro_after_init_sections[i]);
 		if (ndx)
 			info->sechdrs[ndx].sh_flags |= SHF_RO_AFTER_INIT;
-- 
2.21.0.392.gf8f6787159e-goog