Re: [PATCH v4 2/6] crypto: Use memzero_explicit() for clearing state

[Ard Biesheuvel]

On Sun, 25 Oct 2020 at 15:31, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> Without the barrier_data() inside memzero_explicit(), the compiler may
> optimize away the state-clearing if it can tell that the state is not
> used afterwards.
>
> Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>

Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

I agree with Eric that, even though there are cases where it is
unlikely that the compiler could elide an ordinary memset() or struct
assignment (even under LTO), using memzero_explicit() is better in
these cases, as it also clarifies the intent of the operation, and
doesn't result in worse code now that memzero_explicit() is a static
inline around memset() and a barrier.





> ---
>  arch/arm64/crypto/ghash-ce-glue.c | 2 +-
>  arch/arm64/crypto/poly1305-glue.c | 2 +-
>  arch/arm64/crypto/sha3-ce-glue.c  | 2 +-
>  arch/x86/crypto/poly1305_glue.c   | 2 +-
>  include/crypto/sha1_base.h        | 3 ++-
>  include/crypto/sha256_base.h      | 3 ++-
>  include/crypto/sha512_base.h      | 3 ++-
>  include/crypto/sm3_base.h         | 3 ++-
>  8 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/arch/arm64/crypto/ghash-ce-glue.c b/arch/arm64/crypto/ghash-ce-glue.c
> index 8536008e3e35..2427e2f3a9a1 100644
> --- a/arch/arm64/crypto/ghash-ce-glue.c
> +++ b/arch/arm64/crypto/ghash-ce-glue.c
> @@ -168,7 +168,7 @@ static int ghash_final(struct shash_desc *desc, u8 *dst)
>         put_unaligned_be64(ctx->digest[1], dst);
>         put_unaligned_be64(ctx->digest[0], dst + 8);
>
> -       *ctx = (struct ghash_desc_ctx){};
> +       memzero_explicit(ctx, sizeof(*ctx));
>         return 0;
>  }
>
> diff --git a/arch/arm64/crypto/poly1305-glue.c b/arch/arm64/crypto/poly1305-glue.c
> index f33ada70c4ed..683de671741a 100644
> --- a/arch/arm64/crypto/poly1305-glue.c
> +++ b/arch/arm64/crypto/poly1305-glue.c
> @@ -177,7 +177,7 @@ void poly1305_final_arch(struct poly1305_desc_ctx *dctx, u8 *dst)
>         }
>
>         poly1305_emit(&dctx->h, dst, dctx->s);
> -       *dctx = (struct poly1305_desc_ctx){};
> +       memzero_explicit(dctx, sizeof(*dctx));
>  }
>  EXPORT_SYMBOL(poly1305_final_arch);
>
> diff --git a/arch/arm64/crypto/sha3-ce-glue.c b/arch/arm64/crypto/sha3-ce-glue.c
> index 9a4bbfc45f40..e5a2936f0886 100644
> --- a/arch/arm64/crypto/sha3-ce-glue.c
> +++ b/arch/arm64/crypto/sha3-ce-glue.c
> @@ -94,7 +94,7 @@ static int sha3_final(struct shash_desc *desc, u8 *out)
>         if (digest_size & 4)
>                 put_unaligned_le32(sctx->st[i], (__le32 *)digest);
>
> -       *sctx = (struct sha3_state){};
> +       memzero_explicit(sctx, sizeof(*sctx));
>         return 0;
>  }
>
> diff --git a/arch/x86/crypto/poly1305_glue.c b/arch/x86/crypto/poly1305_glue.c
> index e508dbd91813..64d09520d279 100644
> --- a/arch/x86/crypto/poly1305_glue.c
> +++ b/arch/x86/crypto/poly1305_glue.c
> @@ -209,7 +209,7 @@ void poly1305_final_arch(struct poly1305_desc_ctx *dctx, u8 *dst)
>         }
>
>         poly1305_simd_emit(&dctx->h, dst, dctx->s);
> -       *dctx = (struct poly1305_desc_ctx){};
> +       memzero_explicit(dctx, sizeof(*dctx));
>  }
>  EXPORT_SYMBOL(poly1305_final_arch);
>
> diff --git a/include/crypto/sha1_base.h b/include/crypto/sha1_base.h
> index 20fd1f7468af..a5d6033efef7 100644
> --- a/include/crypto/sha1_base.h
> +++ b/include/crypto/sha1_base.h
> @@ -12,6 +12,7 @@
>  #include <crypto/sha.h>
>  #include <linux/crypto.h>
>  #include <linux/module.h>
> +#include <linux/string.h>
>
>  #include <asm/unaligned.h>
>
> @@ -101,7 +102,7 @@ static inline int sha1_base_finish(struct shash_desc *desc, u8 *out)
>         for (i = 0; i < SHA1_DIGEST_SIZE / sizeof(__be32); i++)
>                 put_unaligned_be32(sctx->state[i], digest++);
>
> -       *sctx = (struct sha1_state){};
> +       memzero_explicit(sctx, sizeof(*sctx));
>         return 0;
>  }
>
> diff --git a/include/crypto/sha256_base.h b/include/crypto/sha256_base.h
> index 6ded110783ae..93f9fd21cc06 100644
> --- a/include/crypto/sha256_base.h
> +++ b/include/crypto/sha256_base.h
> @@ -12,6 +12,7 @@
>  #include <crypto/sha.h>
>  #include <linux/crypto.h>
>  #include <linux/module.h>
> +#include <linux/string.h>
>
>  #include <asm/unaligned.h>
>
> @@ -105,7 +106,7 @@ static inline int sha256_base_finish(struct shash_desc *desc, u8 *out)
>         for (i = 0; digest_size > 0; i++, digest_size -= sizeof(__be32))
>                 put_unaligned_be32(sctx->state[i], digest++);
>
> -       *sctx = (struct sha256_state){};
> +       memzero_explicit(sctx, sizeof(*sctx));
>         return 0;
>  }
>
> diff --git a/include/crypto/sha512_base.h b/include/crypto/sha512_base.h
> index fb19c77494dc..93ab73baa38e 100644
> --- a/include/crypto/sha512_base.h
> +++ b/include/crypto/sha512_base.h
> @@ -12,6 +12,7 @@
>  #include <crypto/sha.h>
>  #include <linux/crypto.h>
>  #include <linux/module.h>
> +#include <linux/string.h>
>
>  #include <asm/unaligned.h>
>
> @@ -126,7 +127,7 @@ static inline int sha512_base_finish(struct shash_desc *desc, u8 *out)
>         for (i = 0; digest_size > 0; i++, digest_size -= sizeof(__be64))
>                 put_unaligned_be64(sctx->state[i], digest++);
>
> -       *sctx = (struct sha512_state){};
> +       memzero_explicit(sctx, sizeof(*sctx));
>         return 0;
>  }
>
> diff --git a/include/crypto/sm3_base.h b/include/crypto/sm3_base.h
> index 1cbf9aa1fe52..2f3a32ab97bb 100644
> --- a/include/crypto/sm3_base.h
> +++ b/include/crypto/sm3_base.h
> @@ -13,6 +13,7 @@
>  #include <crypto/sm3.h>
>  #include <linux/crypto.h>
>  #include <linux/module.h>
> +#include <linux/string.h>
>  #include <asm/unaligned.h>
>
>  typedef void (sm3_block_fn)(struct sm3_state *sst, u8 const *src, int blocks);
> @@ -104,7 +105,7 @@ static inline int sm3_base_finish(struct shash_desc *desc, u8 *out)
>         for (i = 0; i < SM3_DIGEST_SIZE / sizeof(__be32); i++)
>                 put_unaligned_be32(sctx->state[i], digest++);
>
> -       *sctx = (struct sm3_state){};
> +       memzero_explicit(sctx, sizeof(*sctx));
>         return 0;
>  }
>
> --
> 2.26.2
>