Subject: Re: linux-next: Tree for Jan 20 -- Kernel panic - Unable to mount root fs

From: Paul Moore <[email-masked]>
Date: Wed, 21 Jan 2015 21:28:51 -0500

On Wednesday, January 21, 2015 09:28:33 PM Al Viro wrote:
> On Wed, Jan 21, 2015 at 01:03:20PM -0800, Guenter Roeck wrote:
> > failing case:
> > 
> > path_lookupat: calling path_init 'usr' flags=40
> > path_init: link_path_walk() returned 0
> > path_lookupat: path_init 'usr' flags=40[50] returned 0
> > walk_component: lookup_fast() returned 1
> > walk_component: lookup_slow() returned 0
> > walk_component: inode=  (null), negative=1
> > do_path_lookup(usr, 0x10)
> > path_lookupat: calling path_init 'usr' flags=50
> > path_init: link_path_walk() returned 0
> > path_lookupat: path_init 'usr' flags=50[50] returned 0
> > mkdir[c74012a0,/kkk] => 0						<==== SIC!
> 
> Cute. 'k' being 0x6b, aka POISON_FREE...  OK, the next question is what's
> been freed under us - I don't believe that it's dentry itself...
> Oh, fuck.  OK, I see what happens.  Look at kern_path_create(); it does
> LOOKUP_PARENT walk, leaving nd->last pointing to the last component of
> the *COPY* of the name it's just created, walked and freed.
> 
> OK...  Fortunately, struct nameidata is completely opaque outside of
> fs/namei.c, so we only need to care about a couple of codepaths.
> 
> Folks, could you check if the following on top of linux-next fixes the
> problem?

Thanks Al, and Guenter, and Sabrina for helping resolve this; it surely would 
have taken me much longer alone.

Al, do you mind if I fold your patch below into the existing patches?
 
> diff --git a/fs/namei.c b/fs/namei.c
> index 323957f..cda89c3 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2056,13 +2056,22 @@ static int do_path_lookup(int dfd, const char *name,
> /* does lookup, returns the object with parent locked */
>  struct dentry *kern_path_locked(const char *name, struct path *path)
>  {
> +	struct filename *filename = getname_kernel(name);
>  	struct nameidata nd;
>  	struct dentry *d;
> -	int err = do_path_lookup(AT_FDCWD, name, LOOKUP_PARENT, &nd);
> -	if (err)
> +	int err;
> +
> +	if (IS_ERR(filename))
> +		return ERR_CAST(filename);
> +
> +	err = filename_lookup(AT_FDCWD, filename, LOOKUP_PARENT, &nd);
> +	if (err) {
> +		putname(filename);
>  		return ERR_PTR(err);
> +	}
>  	if (nd.last_type != LAST_NORM) {
>  		path_put(&nd.path);
> +		putname(filename);
>  		return ERR_PTR(-EINVAL);
>  	}
>  	mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
> @@ -2070,9 +2079,11 @@ struct dentry *kern_path_locked(const char *name,
> struct path *path) if (IS_ERR(d)) {
>  		mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
>  		path_put(&nd.path);
> +		putname(filename);
>  		return d;
>  	}
>  	*path = nd.path;
> +	putname(filename);
>  	return d;
>  }
> 
> @@ -3314,7 +3325,7 @@ struct file *do_file_open_root(struct dentry *dentry,
> struct vfsmount *mnt, return file;
>  }
> 
> -struct dentry *kern_path_create(int dfd, const char *pathname,
> +static struct dentry *filename_create(int dfd, struct filename *name,
>  				struct path *path, unsigned int lookup_flags)
>  {
>  	struct dentry *dentry = ERR_PTR(-EEXIST);
> @@ -3329,7 +3340,7 @@ struct dentry *kern_path_create(int dfd, const char
> *pathname, */
>  	lookup_flags &= LOOKUP_REVAL;
> 
> -	error = do_path_lookup(dfd, pathname, LOOKUP_PARENT|lookup_flags, &nd);
> +	error = filename_lookup(dfd, name, LOOKUP_PARENT|lookup_flags, &nd);
>  	if (error)
>  		return ERR_PTR(error);
> 
> @@ -3383,6 +3394,19 @@ out:
>  	path_put(&nd.path);
>  	return dentry;
>  }
> +
> +struct dentry *kern_path_create(int dfd, const char *pathname,
> +				struct path *path, unsigned int lookup_flags)
> +{
> +	struct filename *filename = getname_kernel(pathname);
> +	struct dentry *res = ERR_CAST(filename);
> +
> +	if (!IS_ERR(filename)) {
> +		res = filename_create(dfd, filename, path, lookup_flags);
> +		putname(filename);
> +	}
> +	return res;
> +}
>  EXPORT_SYMBOL(kern_path_create);
> 
>  void done_path_create(struct path *path, struct dentry *dentry)
> @@ -3401,7 +3425,7 @@ struct dentry *user_path_create(int dfd, const char
> __user *pathname, struct dentry *res;
>  	if (IS_ERR(tmp))
>  		return ERR_CAST(tmp);
> -	res = kern_path_create(dfd, tmp->name, path, lookup_flags);
> +	res = filename_create(dfd, tmp, path, lookup_flags);
>  	putname(tmp);
>  	return res;
>  }

-- 
paul moore
security @ redhat