Re: [PATCH 2/2] staging/lustre/llite: fix ll_getname user buffer copy

[Dan Carpenter <dan.carpenter@oracle.com>]

On Wed, Jun 10, 2015 at 12:41:23AM -0400, green@linuxhacker.ru wrote:
> From: Oleg Drokin <green@linuxhacker.ru>
> 
> strncpy_from_user could return negative values on error,
> so need to take those into account.
> Since ll_getname is used to get a single component name from userspace
> to transfer to server as-is, there's no need to allocate 4k buffer
> as done by __getname. Allocate NAME_MAX+1 buffer instead to ensure
> we have enough for a null terminated max valid length buffer.
> 
> This was discovered by Al Viro in https://lkml.org/lkml/2015/4/11/243
> 
> Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
> ---
>  drivers/staging/lustre/lustre/llite/dir.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/staging/lustre/lustre/llite/dir.c b/drivers/staging/lustre/lustre/llite/dir.c
> index 87a042c..e0b9043 100644
> --- a/drivers/staging/lustre/lustre/llite/dir.c
> +++ b/drivers/staging/lustre/lustre/llite/dir.c
> @@ -1213,29 +1213,31 @@ out:
>  	return rc;
>  }
>  
> -static char *
> -ll_getname(const char __user *filename)
> +/* This function tries to get a single name component,
> + * to send to the server. No actual path traversal involved,
> + * so we limit to NAME_MAX */
> +static char *ll_getname(const char __user *filename)
>  {
>  	int ret = 0, len;
> -	char *tmp = __getname();
> +	char *tmp = kzalloc(NAME_MAX + 1, GFP_KERNEL);

Doing allocations in the declaration block is rare in the kernel but it
accounts for around a quarter of the missing NULL checks and many memory
leaks in the kbuild zero day bot testing.  It's a bad idea and some
subsystems ban the practice, but Greg is fine with it so I'm not going
to complain.

This is me keeping totally silent like a mouse.  :P

>  
>  	if (!tmp)
>  		return ERR_PTR(-ENOMEM);
>  
> -	len = strncpy_from_user(tmp, filename, PATH_MAX);
> -	if (len == 0)
> +	len = strncpy_from_user(tmp, filename, NAME_MAX);
> +	if (len < 0)
> +		ret = len;
> +	else if (len == 0)
>  		ret = -ENOENT;
> -	else if (len > PATH_MAX)
> -		ret = -ENAMETOOLONG;

I don't like how this does silent truncation.  strncpy_from_user()
return -EFAULT if we run into unmapped memory.  Otherwise if the user
supplies a too long name it returns len == PATH_MAX.  (I think, the
documentation for this function is hard to understand).

Of course, the check was never true in the original code...

regards,
dan carpenter