dh key: fix rounding up KDF output length

Commit 383203ef ("dh key: get rid of stack allocated array") changed
kdf_ctr() to assume that the length of key material to derive is a
multiple of the digest size.  The length was supposed to be rounded up
accordingly.  However, the round_up() macro was used which only gives
the correct result on power-of-2 arguments, whereas not all hash
algorithms have power-of-2 digest sizes.  In some cases this resulted in
a write past the end of the 'outbuf' buffer.

Fix it by switching to roundup(), which works for non-power-of-2 inputs.

Reported-by: syzbot+486f97f892efeb2075a3@syzkaller.appspotmail.com
Reported-by: syzbot+29d17b7898b41ee120a5@syzkaller.appspotmail.com
Reported-by: syzbot+8a608baf8751184ec727@syzkaller.appspotmail.com
Reported-by: syzbot+d04e58bd384f1fe0b112@syzkaller.appspotmail.com
Fixes: 383203ef ("dh key: get rid of stack allocated array")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: James Morris <james.morris@microsoft.com>

security/keys/dh.c

@@ -142,6 +142,8 @@ static void kdf_dealloc(struct kdf_sdesc *sdesc)
  * The src pointer is defined as Z || other info where Z is the shared secret
  * from DH and other info is an arbitrary string (see SP800-56A section
  * 5.8.1.2).
+ *
+ * 'dlen' must be a multiple of the digest size.
  */
 static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 		   u8 *dst, unsigned int dlen, unsigned int zlen)
@@ -205,7 +207,7 @@ static int keyctl_dh_compute_kdf(struct kdf_sdesc *sdesc,
 {
 	uint8_t *outbuf = NULL;
 	int ret;
-	size_t outbuf_len = round_up(buflen,
+	size_t outbuf_len = roundup(buflen,
 				    crypto_shash_digestsize(sdesc->shash.tfm));
 
 	outbuf = kmalloc(outbuf_len, GFP_KERNEL);