devpts: if initialization failed, don't crash when opening /dev/ptmx

If devpts failed to initialize, it would store an ERR_PTR in the global
devpts_mnt.  A subsequent open of /dev/ptmx would call devpts_new_index,
which would dereference devpts_mnt and crash.

Avoid storing invalid values in devpts_mnt; leave it NULL instead.  Make
both devpts_new_index and devpts_pty_new fail gracefully with ENODEV in
that case, which then becomes the return value to the userspace open call
on /dev/ptmx.

[[email protected]: remove unneeded static]
Signed-off-by: Josh Triplett <[email protected]>
Reported-by: Fengguang Wu <[email protected]>
Reviewed-by: Peter Hurley <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/devpts/inode.c

@@ -142,6 +142,8 @@ static inline struct super_block *pts_sb_from_inode(struct inode *inode)
  if (inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
  return inode->i_sb;
 #endif
+ if (!devpts_mnt)
+ return NULL;
  return devpts_mnt->mnt_sb;
 }
 
@@ -525,10 +527,14 @@ static struct file_system_type devpts_fs_type = {
 int devpts_new_index(struct inode *ptmx_inode)
 {
  struct super_block *sb = pts_sb_from_inode(ptmx_inode);
- struct pts_fs_info *fsi = DEVPTS_SB(sb);
+ struct pts_fs_info *fsi;
  int index;
  int ida_ret;
 
+ if (!sb)
+ return -ENODEV;
+
+ fsi = DEVPTS_SB(sb);
 retry:
  if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
  return -ENOMEM;
@@ -584,11 +590,18 @@ struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
  struct dentry *dentry;
  struct super_block *sb = pts_sb_from_inode(ptmx_inode);
  struct inode *inode;
- struct dentry *root = sb->s_root;
- struct pts_fs_info *fsi = DEVPTS_SB(sb);
- struct pts_mount_opts *opts = &fsi->mount_opts;
+ struct dentry *root;
+ struct pts_fs_info *fsi;
+ struct pts_mount_opts *opts;
  char s[12];
 
+ if (!sb)
+ return ERR_PTR(-ENODEV);
+
+ root = sb->s_root;
+ fsi = DEVPTS_SB(sb);
+ opts = &fsi->mount_opts;
+
  inode = new_inode(sb);
  if (!inode)
  return ERR_PTR(-ENOMEM);
@@ -676,12 +689,16 @@ static int __init init_devpts_fs(void)
  struct ctl_table_header *table;
 
  if (!err) {
+ struct vfsmount *mnt;
+
  table = register_sysctl_table(pty_root_table);
- devpts_mnt = kern_mount(&devpts_fs_type);
- if (IS_ERR(devpts_mnt)) {
- err = PTR_ERR(devpts_mnt);
+ mnt = kern_mount(&devpts_fs_type);
+ if (IS_ERR(mnt)) {
+ err = PTR_ERR(mnt);
  unregister_filesystem(&devpts_fs_type);
  unregister_sysctl_table(table);
+ } else {
+ devpts_mnt = mnt;
  }
  }
  return err;