linux-kernel - Re: [PATCH v4 02/29] rxrpc: Avoid using stack memory in SG lists in rxkad

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Date:	Tue, 28 Jun 2016 08:52:20 +0100
From:	David Howells <dhowells@...hat.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	dhowells@...hat.com, x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH v4 02/29] rxrpc: Avoid using stack memory in SG lists in rxkad

Andy Lutomirski <luto@...nel.org> wrote:

> -	skcipher_request_set_crypt(req, &sg[1], &sg[0], sizeof(tmpbuf), iv.x);
> +	skcipher_request_set_crypt(req, &sg, &sg, sizeof(tmpbuf), iv.x);

Don't the sg's have to be different?  Aren't they both altered by the process
of reading/writing from them?

> 	struct rxrpc_skb_priv *sp;
> ...
> +	swap(tmpbuf.xl, *(__be64 *)sp);
> +
> +	sg_init_one(&sg, sp, sizeof(tmpbuf));

????  I assume you're assuming that the rxrpc_skb_priv struct contents can
arbitrarily replaced temporarily...

And using an XCHG-equivalent instruction?  This won't work on a 32-bit arch
(apart from one that sports CMPXCHG8 or similar).

>  /*
> - * load a scatterlist with a potentially split-page buffer
> + * load a scatterlist
>   */
> -static void rxkad_sg_set_buf2(struct scatterlist sg[2],
> +static void rxkad_sg_set_buf2(struct scatterlist sg[1],
>  			      void *buf, size_t buflen)
>  {
> -	int nsg = 1;
> -
> -	sg_init_table(sg, 2);
> -
> +	sg_init_table(sg, 1);
>  	sg_set_buf(&sg[0], buf, buflen);
> -	if (sg[0].offset + buflen > PAGE_SIZE) {
> -		/* the buffer was split over two pages */
> -		sg[0].length = PAGE_SIZE - sg[0].offset;
> -		sg_set_buf(&sg[1], buf + sg[0].length, buflen - sg[0].length);
> -		nsg++;
> -	}
> -
> -	sg_mark_end(&sg[nsg - 1]);
> -
> -	ASSERTCMP(sg[0].length + sg[1].length, ==, buflen);
>  }

This should be a separate patch.

David