userswap: add a kernel parameter to enable userswap

hulk inclusion
category: bugfix
bugzilla: 175146
CVE: NA

------------------------------------

Disable userswap by default and add a kernel parameter to enable it.
Signed-off-by: NXiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/userfaultfd.c

@@ -31,6 +31,9 @@
 #include <linux/hugetlb.h>
 
 static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
+#ifdef CONFIG_USERSWAP
+int enable_userswap;
+#endif
 
 enum userfaultfd_state {
 	UFFD_STATE_WAIT_API,
@@ -906,7 +909,8 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		userfault_flags = VM_UFFD_MISSING | VM_UFFD_WP;
 #ifdef CONFIG_USERSWAP
-		userfault_flags |= VM_USWAP;
+		if (enable_userswap)
+			userfault_flags |= VM_USWAP;
 #endif
 		cond_resched();
 		BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
@@ -1335,7 +1339,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 	 * register the whole vma overlapping with the address range to avoid
 	 * splitting the vma.
 	 */
-	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_USWAP) {
+	if (enable_userswap && (uffdio_register.mode & UFFDIO_REGISTER_MODE_USWAP)) {
 		uffdio_register.mode &= ~UFFDIO_REGISTER_MODE_USWAP;
 		if (!uffdio_register.mode)
 			goto out;
@@ -2008,6 +2012,15 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	return fd;
 }
 
+#ifdef CONFIG_USERSWAP
+static int __init enable_userswap_setup(char *str)
+{
+	enable_userswap = true;
+	return 1;
+}
+__setup("enable_userswap", enable_userswap_setup);
+#endif
+
 static int __init userfaultfd_init(void)
 {
 	userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",

include/linux/userfaultfd_k.h

@@ -28,6 +28,10 @@
 #define UFFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
 #define UFFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS)
 
+#ifdef CONFIG_USERSWAP
+extern int enable_userswap;
+#endif
+
 extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
 
 extern ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,

mm/mmap.c

@@ -1580,7 +1580,7 @@ unsigned long __do_mmap(struct mm_struct *mm, struct file *file,
 			prot |= PROT_EXEC;
 
 #ifdef CONFIG_USERSWAP
-	if (flags & MAP_REPLACE) {
+	if (enable_userswap && (flags & MAP_REPLACE)) {
 		if (offset_in_page(addr) || (len % PAGE_SIZE))
 			return -EINVAL;
 		page_num = len / PAGE_SIZE;
@@ -1768,7 +1768,7 @@ unsigned long __do_mmap(struct mm_struct *mm, struct file *file,
 
 #ifdef CONFIG_USERSWAP
 	/* mark the vma as special to avoid merging with other vmas */
-	if (flags & MAP_REPLACE)
+	if (enable_userswap && (flags & MAP_REPLACE))
 		vm_flags |= VM_SPECIAL;
 #endif
 
@@ -1780,7 +1780,7 @@ unsigned long __do_mmap(struct mm_struct *mm, struct file *file,
 #ifndef CONFIG_USERSWAP
 	return addr;
 #else
-	if (!(flags & MAP_REPLACE))
+	if (!enable_userswap || !(flags & MAP_REPLACE))
 		return addr;
 
 	if (IS_ERR_VALUE(addr)) {