Re: [PATCH] devpts: If initialization failed, don't crash when opening /dev/ptmx

Peter Hurley
2015-05-07T17:52:50Z

On 05/06/2015 08:35 PM, Josh Triplett wrote:
> 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.
>
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>
> ---
>
> This fixes a crash found by Fengguang Wu's 0-day service ("BUG: unable to
> handle kernel paging request at ffffffee"). It doesn't yet fix the underlying
> initialization failure in init_devpts_fs, but it stops that failure from
> becoming a kernel crash. I'm working on the initialization failure now.
>
> fs/devpts/inode.c | 30 +++++++++++++++++++++++-------
> 1 file changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index cfe8466..03e9076 100644
> --- a/fs/devpts/inode.c
> +++ b/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 +689,15 @@ static int __init init_devpts_fs(void)
> struct ctl_table_header *table;
>
> if (!err) {
> + static struct vfsmount *mnt;
^^^^^^
Not static storage. Other than that,

Reviewed-by: Peter Hurley <peter@hurleysoftware.com>

> 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;
>