safesetid: check size of policy writes

syzbot attempts to write a buffer with a large size to a sysfs entry
with writes handled by handle_policy_update(), triggering a warning
in kmalloc.

Check the size specified for write buffers before allocating.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=4eb7a741b3216020043a
Signed-off-by: Leo Stone <[email protected]>
[PM: subject tweak]
Signed-off-by: Paul Moore <[email protected]>

Author: leocstone

security/safesetid/securityfs.c

@@ -143,6 +143,9 @@ static ssize_t handle_policy_update(struct file *file,
 	char *buf, *p, *end;
 	int err;
 
+	if (len >= KMALLOC_MAX_SIZE)
+		return -EINVAL;
+
 	pol = kmalloc(sizeof(struct setid_ruleset), GFP_KERNEL);
 	if (!pol)
 		return -ENOMEM;