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>

4 files changed, 20 insertions, 19 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index c2964d890c9a..8e654468ab67 100644
--- a/fs/proc/base.c
+++ b/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 @@ skip_argv:
                         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;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ecc4be7b67e0..f31bf9058587 100644
--- a/include/linux/mm.h
+++ b/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,
diff --git a/mm/memory.c b/mm/memory.c
index 79ebed3a4c2b..bac2d994850e 100644
--- a/mm/memory.c
+++ b/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);
 }
 
 /*
diff --git a/mm/nommu.c b/mm/nommu.c
index bde7df35118b..93d5bb53fc63 100644
--- a/mm/nommu.c
+++ b/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);
 }
 
 /*