mm: replace access_remote_vm() write parameter with gup_flags

This removes the 'write' argument from access_remote_vm() and replaces
it with 'gup_flags' as use of this function previously silently implied
FOLL_FORCE, whereas after this patch callers explicitly pass this flag.

We make this explicit as use of FOLL_FORCE can result in surprising
behaviour (and hence bugs) within the mm subsystem.
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -252,7 +252,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 	 * Inherently racy -- command line shares address space
 	 * with code and data.
 	 */
-	rv = access_remote_vm(mm, arg_end - 1, &c, 1, 0);
+	rv = access_remote_vm(mm, arg_end - 1, &c, 1, FOLL_FORCE);
 	if (rv <= 0)
 		goto out_free_page;
 
@@ -270,7 +270,8 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			int nr_read;
 
 			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count, 0);
+			nr_read = access_remote_vm(mm, p, page, _count,
+					FOLL_FORCE);
 			if (nr_read < 0)
 				rv = nr_read;
 			if (nr_read <= 0)
@@ -305,7 +306,8 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			bool final;
 
 			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count, 0);
+			nr_read = access_remote_vm(mm, p, page, _count,
+					FOLL_FORCE);
 			if (nr_read < 0)
 				rv = nr_read;
 			if (nr_read <= 0)
@@ -354,7 +356,8 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			bool final;
 
 			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count, 0);
+			nr_read = access_remote_vm(mm, p, page, _count,
+					FOLL_FORCE);
 			if (nr_read < 0)
 				rv = nr_read;
 			if (nr_read <= 0)
@@ -832,6 +835,7 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 	unsigned long addr = *ppos;
 	ssize_t copied;
 	char *page;
+	unsigned int flags = FOLL_FORCE;
 
 	if (!mm)
 		return 0;
@@ -844,6 +848,9 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 	if (!atomic_inc_not_zero(&mm->mm_users))
 		goto free;
 
+	if (write)
+		flags |= FOLL_WRITE;
+
 	while (count > 0) {
 		int this_len = min_t(int, count, PAGE_SIZE);
 
@@ -852,7 +859,7 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 			break;
 		}
 
-		this_len = access_remote_vm(mm, addr, page, this_len, write);
+		this_len = access_remote_vm(mm, addr, page, this_len, flags);
 		if (!this_len) {
 			if (!copied)
 				copied = -EIO;
@@ -965,7 +972,7 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 		this_len = min(max_len, this_len);
 
 		retval = access_remote_vm(mm, (env_start + src),
-			page, this_len, 0);
+			page, this_len, FOLL_FORCE);
 
 		if (retval <= 0) {
 			ret = retval;

include/linux/mm.h

@@ -1268,7 +1268,7 @@ static inline int fixup_user_fault(struct task_struct *tsk,
 
 extern int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len, int write);
 extern int access_remote_vm(struct mm_struct *mm, unsigned long addr,
-		void *buf, int len, int write);
+		void *buf, int len, unsigned int gup_flags);
 
 long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		      unsigned long start, unsigned long nr_pages,

mm/memory.c

@@ -3935,19 +3935,14 @@ static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
  * @addr:	start address to access
  * @buf:	source or destination buffer
  * @len:	number of bytes to transfer
- * @write:	whether the access is a write
+ * @gup_flags:	flags modifying lookup behaviour
  *
  * The caller must hold a reference on @mm.
  */
 int access_remote_vm(struct mm_struct *mm, unsigned long addr,
-		void *buf, int len, int write)
+		void *buf, int len, unsigned int gup_flags)
 {
-	unsigned int flags = FOLL_FORCE;
-
-	if (write)
-		flags |= FOLL_WRITE;
-
-	return __access_remote_vm(NULL, mm, addr, buf, len, flags);
+	return __access_remote_vm(NULL, mm, addr, buf, len, gup_flags);
 }
 
 /*

mm/nommu.c

@@ -1847,15 +1847,14 @@ static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
  * @addr:	start address to access
  * @buf:	source or destination buffer
  * @len:	number of bytes to transfer
- * @write:	whether the access is a write
+ * @gup_flags:	flags modifying lookup behaviour
  *
  * The caller must hold a reference on @mm.
  */
 int access_remote_vm(struct mm_struct *mm, unsigned long addr,
-		void *buf, int len, int write)
+		void *buf, int len, unsigned int gup_flags)
 {
-	return __access_remote_vm(NULL, mm, addr, buf, len,
-			write ? FOLL_WRITE : 0);
+	return __access_remote_vm(NULL, mm, addr, buf, len, gup_flags);
 }
 
 /*