linux-kernel - Re: [PATCH 4.19 287/346] crypto: ecdh - avoid unaligned accesses in ecdh_set

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Date:   Thu, 31 Dec 2020 21:09:13 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 4.19 287/346] crypto: ecdh - avoid unaligned accesses in
 ecdh_set_secret()

Hi!

> ecdh_set_secret() casts a void* pointer to a const u64* in order to
> feed it into ecc_is_key_valid(). This is not generally permitted by
> the C standard, and leads to actual misalignment faults on ARMv6
> cores. In some cases, these are fixed up in software, but this still
> leads to performance hits that are entirely avoidable.
> 
> So let's copy the key into the ctx buffer first, which we will do
> anyway in the common case, and which guarantees correct alignment.

Fair enough... but: params.key_size is validated in
ecc_is_key_valid(), and that now happens _after_ memcpy.

How is it guaranteed that we don't overflow the buffer during memcpy?

> +++ b/crypto/ecdh.c
> @@ -57,12 +57,13 @@ static int ecdh_set_secret(struct crypto
>  		return ecc_gen_privkey(ctx->curve_id, ctx->ndigits,
>  				       ctx->private_key);
>  
> -	if (ecc_is_key_valid(ctx->curve_id, ctx->ndigits,
> -			     (const u64 *)params.key, params.key_size) < 0)
> -		return -EINVAL;
> -
>  	memcpy(ctx->private_key, params.key, params.key_size);
>  
> +	if (ecc_is_key_valid(ctx->curve_id, ctx->ndigits,
> +			     ctx->private_key, params.key_size) < 0) {
> +		memzero_explicit(ctx->private_key, params.key_size);
> +		return -EINVAL;
> +	}
>  	return 0;

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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