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 10:07:44 +0100
From:	David Howells <dhowells@...hat.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	dhowells@...hat.com, Andy Lutomirski <luto@...nel.org>,
	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>
Subject: Re: [PATCH v4 02/29] rxrpc: Avoid using stack memory in SG lists in rxkad

Herbert Xu <herbert@...dor.apana.org.au> wrote:

> Huh? If you can't write to csum_iv_head without clobbering others
> then by the same reasoning you can't write to csum_iv either.  So
> unless you're saying the existing code is already broken then there
> is nothing wrong with the patch.

Ah, for some reason I read it as being in the normal packet processing.  Need
tea before I read security patches;-)

Since it's (more or less) a one off piece of memory, why not kmalloc it
temporarily rather than expanding the connection struct?  Also, the bit where
you put a second rxrpc_crypt in just so that it happens to give you a 16-byte
slot by adjacency is pretty icky.  It would be much better to use a union
instead:

	union {
		struct rxrpc_crypt	csum_iv; /* packet checksum base */
		__be32 tmpbuf[4];
	};

Note also that the above doesn't guarantee that the struct will be inside of a
single page.  It would need an alignment of 16 for that - but you only have
one sg.  Could that be a problem?

David