regset: Prevent null pointer reference on readonly regsets

The regset common infrastructure assumed that regsets would always
have .get and .set methods, but not necessarily .active methods.
Unfortunately people have since written regsets without .set methods.

Rather than putting in stub functions everywhere, handle regsets with
null .get or .set methods explicitly.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@hack.frob.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_elf.c

@@ -1421,7 +1421,7 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 	for (i = 1; i < view->n; ++i) {
 		const struct user_regset *regset = &view->regsets[i];
 		do_thread_regset_writeback(t->task, regset);
-		if (regset->core_note_type &&
+		if (regset->core_note_type && regset->get &&
 		    (!regset->active || regset->active(t->task, regset))) {
 			int ret;
 			size_t size = regset->n * regset->size;

include/linux/regset.h

@@ -335,6 +335,9 @@ static inline int copy_regset_to_user(struct task_struct *target,
 {
 	const struct user_regset *regset = &view->regsets[setno];
 
+	if (!regset->get)
+		return -EOPNOTSUPP;
+
 	if (!access_ok(VERIFY_WRITE, data, size))
 		return -EIO;
 
@@ -358,6 +361,9 @@ static inline int copy_regset_from_user(struct task_struct *target,
 {
 	const struct user_regset *regset = &view->regsets[setno];
 
+	if (!regset->set)
+		return -EOPNOTSUPP;
+
 	if (!access_ok(VERIFY_READ, data, size))
 		return -EIO;