Re: [PATCH] luks2: Fix decoding of digests and salts with escaped chars

[Daniel Kiper]

CC-ing Glenn...

On Wed, Aug 11, 2021 at 08:55:32PM +0200, Patrick Steinhardt wrote:
> It was reported in the #grub IRC channel that decryption of LUKS2

Which IRC network?

> partitions fails with errors about invalid digests and/or salts. In all
> of these cases, what failed was decoding the Base64 representation of
> these, where the encoded data contained invalid characters.
>
> As it turns out, the root cause is that json-c, which is used by
> cryptsetup to read and write the JSON header, will escape some
> characters by prepending a backslash when writing JSON strings. Most
> importantly, this also includes the forward slash, which is part of the

Wait, is not it jsmn lib bug? If it is should not we take updated version
of jsmn? Or if it is not fixed there propose jsmn upstream patch?

> Base64 alphabet and which may optionally be escaped. Because GRUB
> doesn't know to unescape such characters, decoding this string will
> rightfully fail.
>
> Fix the issue by stripping the escape character for forward slashes.
> There is no need to do so for any other escaped character given that
> valid Base64 does not contain anything else.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> Reported-by: Afdal

Reported-by should go before SOB. And it would be nice if you could get
Afdal's email address...

> ---
>  grub-core/disk/luks2.c | 34 ++++++++++++++++++++++++++++++----
>  1 file changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/grub-core/disk/luks2.c b/grub-core/disk/luks2.c
> index 371a53b83..7e596f90d 100644
> --- a/grub-core/disk/luks2.c
> +++ b/grub-core/disk/luks2.c
> @@ -383,6 +383,32 @@ luks2_scan (grub_disk_t disk, const char *check_uuid, 
> int check_boot)
>    return cryptodisk;
>  }
>
> +static int luks2_base64_decode (const char *in, size_t inlen, char *decoded, 
> size_t *decodedlen)

static int
luks2_base64_decode (...

> +{
> +  char *unescaped = NULL;

I think you can drop this initialization.

> +  size_t i, j;
> +  int err;
> +
> +  unescaped = grub_malloc (inlen);

The grub_malloc() may return NULL. Please do not ignore that...

> +  grub_memmove (unescaped, in, inlen);
> +
> +  /*
> +   * JSON strings can escape some characters. Given that we expect
> +   * Base64-encoded input here, the only character from the Base64 alphabet
> +   * which may optionally be escaped is the forward slash. We thus strip away
> +   * the escape character if we see '\/' in the input.
> +   */
> +  for (i = j = 0; i < inlen; i++) {
> +    if (i + 1 < inlen && unescaped[i] == '\\' && unescaped[i + 1] == '/')
> +      continue;
> +    unescaped[j++] = unescaped[i];
> +  }
> +
> +  err = base64_decode (unescaped, j, decoded, decodedlen);
> +  grub_free (unescaped);
> +  return err;
> +}
> +
>  static grub_err_t
>  luks2_verify_key (grub_luks2_digest_t *d, grub_uint8_t *candidate_key,
>                 grub_size_t candidate_key_len)
> @@ -394,9 +420,9 @@ luks2_verify_key (grub_luks2_digest_t *d, grub_uint8_t 
> *candidate_key,
>    gcry_err_code_t gcry_ret;
>
>    /* Decode both digest and salt */
> -  if (!base64_decode (d->digest, grub_strlen (d->digest), (char *)digest, 
> &digestlen))
> +  if (!luks2_base64_decode (d->digest, grub_strlen (d->digest), (char 
> *)digest, &digestlen))
>      return grub_error (GRUB_ERR_BAD_ARGUMENT, "Invalid digest");
> -  if (!base64_decode (d->salt, grub_strlen (d->salt), (char *)salt, 
> &saltlen))
> +  if (!luks2_base64_decode (d->salt, grub_strlen (d->salt), (char *)salt, 
> &saltlen))
>      return grub_error (GRUB_ERR_BAD_ARGUMENT, "Invalid digest salt");
>
>    /* Configure the hash used for the digest. */
> @@ -434,8 +460,8 @@ luks2_decrypt_key (grub_uint8_t *out_key,
>    gcry_err_code_t gcry_ret;
>    grub_err_t ret;
>
> -  if (!base64_decode (k->kdf.salt, grub_strlen (k->kdf.salt),
> -                  (char *)salt, &saltlen))
> +  if (!luks2_base64_decode (k->kdf.salt, grub_strlen (k->kdf.salt),
> +                         (char *)salt, &saltlen))
>      {
>        ret = grub_error (GRUB_ERR_BAD_ARGUMENT, "Invalid keyslot salt");
>        goto err;

Daniel