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

josh
2015-05-07T18:35:00Z

On Thu, May 07, 2015 at 01:52:37PM -0400, Peter Hurley wrote:
> 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>

Gah. I deleted that, but apparently didn't "git add". Good catch,
thanks; v2 momentarily.

> > 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;
> >
>