disk/luks2: Fix decoding of digests and salts with escaped chars

It was reported in the #grub IRC channel on Libera that decryption of
LUKS2 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 by
default. Most importantly, json-c also escapes the forward slash, which
is part of the Base64 alphabet. Because GRUB doesn't know to unescape
such characters, decoding this string will rightfully fail.

Interestingly, this issue has until now only been reported by users of
Ubuntu 18.04. And a bit of digging in fact reveals that cryptsetup has
changed the logic in a054206d (Suppress useless slash escaping in json
lib, 2018-04-20), which has been released with cryptsetup v2.0.3. Ubuntu
18.04 is still shipping with cryptsetup v2.0.2 though, which explains
why this is not a more frequent issue.

Fix the issue by using our new grub_json_unescape() helper function
that handles unescaping for us.

Reported-by: Afdal
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <dkiper@net-space.pl>

1 files changed, 24 insertions, 4 deletions

diff --git a/grub-core/disk/luks2.c b/grub-core/disk/luks2.c
index 5b3b36c8a..e4bef3d0e 100644
--- a/grub-core/disk/luks2.c
+++ b/grub-core/disk/luks2.c
@@ -385,6 +385,24 @@ luks2_scan (grub_disk_t disk, grub_cryptomount_args_t cargs)
 }
 
 static grub_err_t
+luks2_base64_decode (const char *in, grub_size_t inlen, grub_uint8_t *decoded, idx_t *decodedlen)
+{
+  grub_size_t unescaped_len = 0;
+  char *unescaped = NULL;
+  bool successful;
+
+  if (grub_json_unescape (&unescaped, &unescaped_len, in, inlen) != GRUB_ERR_NONE)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("could not unescape Base64 string"));
+
+  successful = base64_decode (unescaped, unescaped_len, (char *) decoded, decodedlen);
+  grub_free (unescaped);
+  if (!successful)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("could not decode Base64 string"));
+
+  return GRUB_ERR_NONE;
+}
+
+static grub_err_t
 luks2_verify_key (grub_luks2_digest_t *d, grub_uint8_t *candidate_key,
 		  grub_size_t candidate_key_len)
 {
@@ -395,9 +413,11 @@ 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),
+			   digest, &digestlen) != GRUB_ERR_NONE)
     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),
+			   salt, &saltlen) != GRUB_ERR_NONE)
     return grub_error (GRUB_ERR_BAD_ARGUMENT, "Invalid digest salt");
 
   /* Configure the hash used for the digest. */
@@ -435,8 +455,8 @@ 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),
+			   salt, &saltlen) != GRUB_ERR_NONE)
     {
       ret = grub_error (GRUB_ERR_BAD_ARGUMENT, "Invalid keyslot salt");
       goto err;