[Qemu-devel] [PATCH v2 2/7] crypto: make PBKDF iterations configurable for LUKS format

[Daniel P. Berrange]

As protection against bruteforcing passphrases, the PBKDF
algorithm is tuned by counting the number of iterations
needed to produce 1 second of running time. If the machine
that the image will be used on is much faster than the
machine where the image is created, it can be desirable
to raise the number of iterations. This change adds a new
'iter-time' property that allows the user to choose the
iteration wallclock time.

Signed-off-by: Daniel P. Berrange <address@hidden>
---
 block/crypto.c      |  6 ++++++
 crypto/block-luks.c | 24 ++++++++++++++++++++++++
 qapi/crypto.json    |  6 +++++-
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/block/crypto.c b/block/crypto.c
index 7f61e12..7aa7eb5 100644
--- a/block/crypto.c
+++ b/block/crypto.c
@@ -33,6 +33,7 @@
 #define BLOCK_CRYPTO_OPT_LUKS_IVGEN_ALG "ivgen-alg"
 #define BLOCK_CRYPTO_OPT_LUKS_IVGEN_HASH_ALG "ivgen-hash-alg"
 #define BLOCK_CRYPTO_OPT_LUKS_HASH_ALG "hash-alg"
+#define BLOCK_CRYPTO_OPT_LUKS_ITER_TIME "iter-time"
 
 typedef struct BlockCrypto BlockCrypto;
 
@@ -183,6 +184,11 @@ static QemuOptsList block_crypto_create_opts_luks = {
             .type = QEMU_OPT_STRING,
             .help = "Name of encryption hash algorithm",
         },
+        {
+            .name = BLOCK_CRYPTO_OPT_LUKS_ITER_TIME,
+            .type = QEMU_OPT_NUMBER,
+            .help = "Time to spend in PBKDF in milliseconds",
+        },
         { /* end of list */ }
     },
 };
diff --git a/crypto/block-luks.c b/crypto/block-luks.c
index bc086ac..91a4172 100644
--- a/crypto/block-luks.c
+++ b/crypto/block-luks.c
@@ -920,6 +920,9 @@ qcrypto_block_luks_create(QCryptoBlock *block,
     uint64_t iters;
 
     memcpy(&luks_opts, &options->u.luks, sizeof(luks_opts));
+    if (!luks_opts.has_iter_time) {
+        luks_opts.iter_time = 1000;
+    }
     if (!luks_opts.has_cipher_alg) {
         luks_opts.cipher_alg = QCRYPTO_CIPHER_ALG_AES_256;
     }
@@ -1075,6 +1078,16 @@ qcrypto_block_luks_create(QCryptoBlock *block,
         goto error;
     }
 
+    if (iters > (ULLONG_MAX / luks_opts.iter_time)) {
+        error_setg_errno(errp, ERANGE,
+                         "PBKDF iterations %llu too large to scale",
+                         (unsigned long long)iters);
+        goto error;
+    }
+
+    /* iter_time was in millis, but count_iters reported for secs */
+    iters = iters * luks_opts.iter_time / 1000;
+
     /* Why /= 8 ?  That matches cryptsetup, but there's no
      * explanation why they chose /= 8... Probably so that
      * if all 8 keyslots are active we only spend 1 second
@@ -1144,6 +1157,17 @@ qcrypto_block_luks_create(QCryptoBlock *block,
         error_propagate(errp, local_err);
         goto error;
     }
+
+    if (iters > (ULLONG_MAX / luks_opts.iter_time)) {
+        error_setg_errno(errp, ERANGE,
+                         "PBKDF iterations %llu too large to scale",
+                         (unsigned long long)iters);
+        goto error;
+    }
+
+    /* iter_time was in millis, but count_iters reported for secs */
+    iters = iters * luks_opts.iter_time / 1000;
+
     /* Why /= 2 ?  That matches cryptsetup, but there's no
      * explanation why they chose /= 2... */
     iters /= 2;
diff --git a/qapi/crypto.json b/qapi/crypto.json
index 34d2583..2b6118f 100644
--- a/qapi/crypto.json
+++ b/qapi/crypto.json
@@ -185,6 +185,9 @@
 #                  Currently defaults to 'sha256'
 # @hash-alg: #optional the master key hash algorithm
 #            Currently defaults to 'sha256'
+# @iter-time: #optional number of milliseconds to spend in
+#             PBKDF passphrase processing. Currently defaults
+#             to 1000. (since 2.8)
 # Since: 2.6
 ##
 { 'struct': 'QCryptoBlockCreateOptionsLUKS',
@@ -193,7 +196,8 @@
             '*cipher-mode': 'QCryptoCipherMode',
             '*ivgen-alg': 'QCryptoIVGenAlgorithm',
             '*ivgen-hash-alg': 'QCryptoHashAlgorithm',
-            '*hash-alg': 'QCryptoHashAlgorithm'}}
+            '*hash-alg': 'QCryptoHashAlgorithm',
+            '*iter-time': 'int'}}
 
 
 ##
-- 
2.7.4