From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4E4BCC48BCF for ; Wed, 9 Jun 2021 22:34:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 28CB3613E7 for ; Wed, 9 Jun 2021 22:34:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229548AbhFIWge (ORCPT ); Wed, 9 Jun 2021 18:36:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:43050 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229534AbhFIWge (ORCPT ); Wed, 9 Jun 2021 18:36:34 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id E84FC613C8; Wed, 9 Jun 2021 22:34:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1623278079; bh=buFRo+uDS2vnOLY9vRPpgCoZ9jSvFr5f8/YIKuV4D90=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=P6pBJ8KcsiOe0uK0uxA81FmuVljxHmAWD1873DnsrdQ9L9yXxrVHpv+nYtX06Mgg3 /+fGg4AVeF9fPBlVr2bK61o+c9j7efsr1Ppa6uz0CeNB616xZdRPyUatKWCgX0b/Rw 1rw9ZwdyanC9DHvWcEzuJzy+BtA+AmMcEojW9E7gGpeewEZqNXADxHYSoa3OHt/HpZ fwN1xHRC6V902E/BbBDaIHsFIVcQBP0So5Us0+LaDFoXVsj7Rz8HO2J6AEK3Xd/Xkf 254+zx7tCTm5XEU2OcjxuBvG4oEr22pDU/vuXAD2nJunUJ5cf7fzmplBhaC7FvlhFK sWjpyp6QX5q4w== Date: Wed, 9 Jun 2021 15:34:37 -0700 From: Eric Biggers To: Ard Biesheuvel Cc: linux-crypto@vger.kernel.org, herbert@gondor.apana.org.au, Sami Tolvanen Subject: Re: [PATCH v2] crypto: shash - avoid comparing pointers to exported functions under CFI Message-ID: References: <20210605065902.53268-1-ardb@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210605065902.53268-1-ardb@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org On Sat, Jun 05, 2021 at 08:59:02AM +0200, Ard Biesheuvel wrote: > crypto_shash_alg_has_setkey() is implemented by testing whether the > .setkey() member of a struct shash_alg points to the default version, > called shash_no_setkey(). As crypto_shash_alg_has_setkey() is a static > inline, this requires shash_no_setkey() to be exported to modules. > > Unfortunately, when building with CFI, function pointers are routed > via CFI stubs which are private to each module (or to the kernel proper) > and so this function pointer comparison may fail spuriously. > > Let's fix this by turning crypto_shash_alg_has_setkey() into an out of > line function, which makes the problem go away. > > Cc: Sami Tolvanen > Signed-off-by: Ard Biesheuvel > --- > v2: add code comment to explain why the function needs to remain out of > line > > crypto/shash.c | 20 +++++++++++++++++--- > include/crypto/internal/hash.h | 8 +------- > 2 files changed, 18 insertions(+), 10 deletions(-) > > diff --git a/crypto/shash.c b/crypto/shash.c > index 2e3433ad9762..36579c37e27d 100644 > --- a/crypto/shash.c > +++ b/crypto/shash.c > @@ -20,12 +20,26 @@ > > static const struct crypto_type crypto_shash_type; > > -int shash_no_setkey(struct crypto_shash *tfm, const u8 *key, > - unsigned int keylen) > +static int shash_no_setkey(struct crypto_shash *tfm, const u8 *key, > + unsigned int keylen) > { > return -ENOSYS; > } > -EXPORT_SYMBOL_GPL(shash_no_setkey); > + > +bool crypto_shash_alg_has_setkey(struct shash_alg *alg) > +{ > + /* > + * Function pointer comparisons such as the one below will not work as > + * expected when CFI is enabled, and the comparison involves an > + * exported symbol: as indirect function calls are routed via CFI stubs > + * that are private to each module, the pointer values may be different > + * even if they refer to the same function. > + * > + * Therefore, this function must remain out of line. > + */ > + return alg->setkey != shash_no_setkey; > +} > +EXPORT_SYMBOL_GPL(crypto_shash_alg_has_setkey); Looks okay, but the comment isn't great since it doesn't get to the point until the last sentence. Also it talks about an exported symbol, which is confusing because shash_no_setkey won't actually be exported anymore. I'd prefer something like the following: /* * Check whether an shash algorithm has a setkey function. * * For CFI compatibility, this must not be an inline function. This is because * when CFI is enabled, modules won't get the same address for shash_no_setkey * (if it were exported, which inlining would require) as the core kernel will. */ bool crypto_shash_alg_has_setkey(struct shash_alg *alg) { return alg->setkey != shash_no_setkey; } EXPORT_SYMBOL_GPL(crypto_shash_alg_has_setkey);