[PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[Eric Biggers]

From: Eric Biggers <ebigg...@google.com>

The result of the Diffie-Hellman computation may be shorter than the
input prime number.  Only calculate the KDF over the actual result;
don't include additional uninitialized memory.

Signed-off-by: Eric Biggers <ebigg...@google.com>
---
 security/keys/dh.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index 1c1cac677041..a3a8607107f5 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -313,17 +313,6 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user 
*params,
                goto error4;
        }
 
-       /*
-        * Concatenate SP800-56A otherinfo past DH shared secret -- the
-        * input to the KDF is (DH shared secret || otherinfo)
-        */
-       if (kdfcopy &&
-           copy_from_user(kbuf + resultlen, kdfcopy->otherinfo,
-                          kdfcopy->otherinfolen) != 0) {
-               ret = -EFAULT;
-               goto error5;
-       }
-
        ret = do_dh(result, base, private, prime);
        if (ret)
                goto error5;
@@ -333,8 +322,17 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user 
*params,
                goto error5;
 
        if (kdfcopy) {
+               /*
+                * Concatenate SP800-56A otherinfo past DH shared secret -- the
+                * input to the KDF is (DH shared secret || otherinfo)
+                */
+               if (copy_from_user(kbuf + nbytes, kdfcopy->otherinfo,
+                                  kdfcopy->otherinfolen) != 0) {
+                       ret = -EFAULT;
+                       goto error5;
+               }
                ret = keyctl_dh_compute_kdf(sdesc, buffer, buflen, kbuf,
-                                           resultlen + kdfcopy->otherinfolen);
+                                           nbytes + kdfcopy->otherinfolen);
        } else {
                ret = nbytes;
                if (copy_to_user(buffer, kbuf, nbytes) != 0)
-- 
2.12.2