proc: do not allow negative offsets on /proc/<pid>/environ

__mem_open() which is called by both /proc/<pid>/environ and
/proc/<pid>/mem ->open() handlers will allow the use of negative offsets.
/proc/<pid>/mem has negative offsets but not /proc/<pid>/environ.

Clean this by moving the 'force FMODE_UNSIGNED_OFFSET flag' to mem_open()
to allow negative offsets only on /proc/<pid>/mem.
Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Brad Spengler <spender@grsecurity.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -695,8 +695,6 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
 		mmput(mm);
 	}
 
-	/* OK to pass negative loff_t, we can catch out-of-range */
-	file->f_mode |= FMODE_UNSIGNED_OFFSET;
 	file->private_data = mm;
 
 	return 0;
@@ -704,7 +702,12 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
 
 static int mem_open(struct inode *inode, struct file *file)
 {
-	return __mem_open(inode, file, PTRACE_MODE_ATTACH);
+	int ret = __mem_open(inode, file, PTRACE_MODE_ATTACH);
+
+	/* OK to pass negative loff_t, we can catch out-of-range */
+	file->f_mode |= FMODE_UNSIGNED_OFFSET;
+
+	return ret;
 }
 
 static ssize_t mem_rw(struct file *file, char __user *buf,