[PATCH 2/2] dh key: get rid of stack array allocation

[Tycho Andersen]

Similarly to the previous patch, we would like to get rid of stack
allocated arrays: https://lkml.org/lkml/2018/3/7/621

In this case, we can also use a malloc style approach to free the temporary
buffer, being careful to also use kzfree to free them (indeed, at least one
of these has a memzero_explicit, but it seems like maybe they both
should?).

Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
CC: David Howells <dhowells@xxxxxxxxxx>
CC: James Morris <jmorris@xxxxxxxxx>
CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
---
 security/keys/dh.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index d1ea9f325f94..f02261b24759 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -162,19 +162,27 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 			goto err;
 
 		if (zlen && h) {
-			u8 tmpbuffer[h];
+			u8 *tmpbuffer;
 			size_t chunk = min_t(size_t, zlen, h);
-			memset(tmpbuffer, 0, chunk);
+
+			err = -ENOMEM;
+			tmpbuffer = kzalloc(chunk, GFP_KERNEL);
+			if (!tmpbuffer)
+				goto err;
 
 			do {
 				err = crypto_shash_update(desc, tmpbuffer,
 							  chunk);
-				if (err)
+				if (err) {
+					kzfree(tmpbuffer);
 					goto err;
+				}
 
 				zlen -= chunk;
 				chunk = min_t(size_t, zlen, h);
 			} while (zlen);
+
+			kzfree(tmpbuffer);
 		}
 
 		if (src && slen) {
@@ -184,13 +192,20 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 		}
 
 		if (dlen < h) {
-			u8 tmpbuffer[h];
+			u8 *tmpbuffer;
+
+			err = -ENOMEM;
+			tmpbuffer = kzalloc(h, GFP_KERNEL);
+			if (!tmpbuffer)
+				goto err;
 
 			err = crypto_shash_final(desc, tmpbuffer);
-			if (err)
+			if (err) {
+				kzfree(tmpbuffer);
 				goto err;
+			}
 			memcpy(dst, tmpbuffer, dlen);
-			memzero_explicit(tmpbuffer, h);
+			kzfree(tmpbuffer);
 			return 0;
 		} else {
 			err = crypto_shash_final(desc, dst);
-- 
2.15.1