proc: make sure mem_open() doesn't pin the target's memory

Once /proc/pid/mem is opened, the memory can't be released until
mem_release() even if its owner exits.

Change mem_open() to do atomic_inc(mm_count) + mmput(), this only
pins mm_struct. Change mem_rw() to do atomic_inc_not_zero(mm_count)
before access_remote_vm(), this verifies that this mm is still alive.

I am not sure what should mem_rw() return if atomic_inc_not_zero()
fails. With this patch it returns zero to match the "mm == NULL" case,
may be it should return -EINVAL like it did before e268337d.

Perhaps it makes sense to add the additional fatal_signal_pending()
check into the main loop, to ensure we do not hold this memory if
the target task was oom-killed.

Cc: stable@kernel.org
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -711,6 +711,13 @@ static int mem_open(struct inode* inode, struct file* file)
 	if (IS_ERR(mm))
 		return PTR_ERR(mm);
 
+	if (mm) {
+		/* ensure this mm_struct can't be freed */
+		atomic_inc(&mm->mm_count);
+		/* but do not pin its memory */
+		mmput(mm);
+	}
+
 	/* OK to pass negative loff_t, we can catch out-of-range */
 	file->f_mode |= FMODE_UNSIGNED_OFFSET;
 	file->private_data = mm;
@@ -734,6 +741,9 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 		return -ENOMEM;
 
 	copied = 0;
+	if (!atomic_inc_not_zero(&mm->mm_users))
+		goto free;
+
 	while (count > 0) {
 		int this_len = min_t(int, count, PAGE_SIZE);
 
@@ -761,6 +771,8 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 	}
 	*ppos = addr;
 
+	mmput(mm);
+free:
 	free_page((unsigned long) page);
 	return copied;
 }
@@ -797,7 +809,7 @@ static int mem_release(struct inode *inode, struct file *file)
 {
 	struct mm_struct *mm = file->private_data;
 	if (mm)
-		mmput(mm);
+		mmdrop(mm);
 	return 0;
 }