Re: [PATCH] nfs client, kernel 2.4.31: readlink result overflow

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

Hi Assar,

On Mon, Sep 12, 2005 at 04:41:19PM -0400, Assar wrote:
> Valdis.Kletnieks@vt.edu writes:
> > > To my reading, the 2.6.13 code does not copy the 4 bytes of length to
> > > rcvbuf.
> > 
> > Hmm... it still does this:
> > 	kaddr[len+rcvbuf->page_base] = '\0';
> > which still has a possible off-by-one? (Was that why you have -1 -4?)
> 
> The check is different.  2.6.13 is using ">=" instead of ">", so hence
> I think that's fine.
> 
> > sizeof(actual_var) is even better, as that way it's clear what you're allowing
> > space for.
> 
> diff -u linux-2.4.31.orig/fs/nfs/nfs2xdr.c linux-2.4.31/fs/nfs/nfs2xdr.c
> --- linux-2.4.31.orig/fs/nfs/nfs2xdr.c	2002-11-28 18:53:15.000000000 -0500
> +++ linux-2.4.31/fs/nfs/nfs2xdr.c	2005-09-12 16:12:30.000000000 -0400
> @@ -571,8 +571,8 @@
>  	strlen = (u32*)kmap(rcvbuf->pages[0]);
>  	/* Convert length of symlink */
>  	len = ntohl(*strlen);
> -	if (len > rcvbuf->page_len)
> -		len = rcvbuf->page_len;
> +	if (len > rcvbuf->page_len - sizeof(*strlen) - 1)
> +		len = rcvbuf->page_len - sizeof(*strlen) - 1;

So the problem is that the "len" variable encapsulated in (u32 *)rcvbuf->pages[0]
does not account for its own length (4 bytes)? 

If thats the reason, you don't need the "-1" there?

Someone with better understanding to ACK this would be nice. Trond?

>  	*strlen = len;
>  	/* NULL terminate the string we got */
>  	string = (char *)(strlen + 1);
> diff -u linux-2.4.31.orig/fs/nfs/nfs3xdr.c linux-2.4.31/fs/nfs/nfs3xdr.c
> --- linux-2.4.31.orig/fs/nfs/nfs3xdr.c	2003-11-28 13:26:21.000000000 -0500
> +++ linux-2.4.31/fs/nfs/nfs3xdr.c	2005-09-12 16:12:29.000000000 -0400
> @@ -759,8 +759,8 @@
>  	strlen = (u32*)kmap(rcvbuf->pages[0]);
>  	/* Convert length of symlink */
>  	len = ntohl(*strlen);
> -	if (len > rcvbuf->page_len)
> -		len = rcvbuf->page_len;
> +	if (len > rcvbuf->page_len - sizeof(*strlen) - 1)
> +		len = rcvbuf->page_len - sizeof(*strlen) - 1;
>  	*strlen = len;
>  	/* NULL terminate the string we got */
>  	string = (char *)(strlen + 1);
-
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