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Date:   Sat, 5 Jun 2021 05:50:45 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-crypto@vger.kernel.org, ebiggers@kernel.org,
        Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH] crypto: shash - stop comparing function pointers to
 avoid breaking CFI
Message-ID: <20210604215045.GA4052@gondor.apana.org.au>
References: <20210604190009.33022-1-ardb@kernel.org>
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On Fri, Jun 04, 2021 at 09:00:09PM +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 <samitolvanen@google.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  crypto/shash.c                 | 11 ++++++++---
>  include/crypto/internal/hash.h |  8 +-------
>  2 files changed, 9 insertions(+), 10 deletions(-)

I think this deserves a comment in the code.

> +bool crypto_shash_alg_has_setkey(struct shash_alg *alg)
> +{
> +       return alg->setkey != shash_no_setkey;
> +}
> +EXPORT_SYMBOL_GPL(crypto_shash_alg_has_setkey);

This would be a good spot for the comment so someone doesn't try
to make it inline again later.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt