dh key: fix rounding up KDF output length

dh key: fix rounding up KDF output length

Commit 383203eff718 ("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: [email protected]
Reported-by: [email protected]
Reported-by: [email protected]
Reported-by: [email protected]
Fixes: 383203eff718 ("dh key: get rid of stack allocated array")
Signed-off-by: Eric Biggers <[email protected]>
Acked-by: Kees Cook <[email protected]>
Acked-by: Tycho Andersen <[email protected]>
Signed-off-by: James Morris <[email protected]>