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

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Date:	Thu, 7 May 2015 11:34:53 -0700
From:	josh@...htriplett.org
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Iulia Manda <iulia.manda21@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Fabian Frederick <fabf@...net.be>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] devpts: If initialization failed, don't crash when
 opening /dev/ptmx

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@...htriplett.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@...leysoftware.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;
> > 
> 
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