[PATCH 3/3] dh key: get rid of stack allocated array for zeroes

[Tycho Andersen]

We're interested in getting rid of all of the stack allocated arrays in
the kernel: https://lkml.org/lkml/2018/3/7/621

This case is interesting, since we really just need an array of bytes that
are zero. The loop already ensures that if the array isn't exactly the
right size that enough zero bytes will be copied in. So, instead of
choosing this value to be the size of the hash, let's just choose it to be
256, since that is a common size, is not to big, and will not result in too
many extra iterations of the loop.

v2: split out from other patch, just hardcode array size instead of
    dynamically allocating something the right size

Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
CC: David Howells <dhowells@xxxxxxxxxx>
CC: James Morris <jmorris@xxxxxxxxx>
CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
CC: Eric Biggers <ebiggers3@xxxxxxxxx>
---
 security/keys/dh.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index 9fecaea6c298..74f8a853872e 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -162,8 +162,8 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 			goto err;
 
 		if (zlen && h) {
-			u8 tmpbuffer[h];
-			size_t chunk = min_t(size_t, zlen, h);
+			u8 tmpbuffer[256];
+			size_t chunk = min_t(size_t, zlen, sizeof(tmpbuffer));
 			memset(tmpbuffer, 0, chunk);
 
 			do {
@@ -173,7 +173,7 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 					goto err;
 
 				zlen -= chunk;
-				chunk = min_t(size_t, zlen, h);
+				chunk = min_t(size_t, zlen, sizeof(tmpbuffer));
 			} while (zlen);
 		}
 
-- 
2.17.0