Re: [PATCH] smb: server: Clear sensitive stack and heap data in auth.c

On 07/08/2026 19.10, Thomas Huth wrote:
> From: Thomas Huth <[email protected]>
> 
> Sensitive data like keys that are stored in stack-local arrays could be
> leaked via the stack to the calling functions, or via the heap when using
> only normal kfree() functions. There is no known vulnaribility for this in
> this code right now, but it's good security style to explicitly zeroize this
> sensitive material as soon as possible to avoid that it could be exploited
> together with other bugs later.
> 
> In calc_ntlmv2_hash(), the struct hmac_md5_ctx is normally cleared during
> hmac_md5_final() already, but in case of errors, this function is skipped
> and ctx is never zeroized, so add a memzero_explicit(&ctx, sizeof(ctx))
> there to fix the problem.
> 
> In ksmbd_krb5_authenticate(), the ksmbd_spnego_authen_response contains
> the session key in the payload. It's currently freed with plain kvfree().
> Let's better use kvfree_sensitive() instead.
> 
> In generate_key(), the prfhash[] array is used to calculate the key,
> but it's never cleared, so it leaks on the stack. Thus clear this with
> a memzero_explicit(), too.
> 
> In ksmbd_crypt_message(), the sign[] and key[] arrays are leaked via
> the stack, too. Make sure to clear them via memzero_explicit() at the
> end.
> 
> Signed-off-by: Thomas Huth <[email protected]>
> ---
>   fs/smb/server/auth.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/smb/server/auth.c b/fs/smb/server/auth.c
> index 4e7b6f0e6b8cd..acdac40bee813 100644
> --- a/fs/smb/server/auth.c
> +++ b/fs/smb/server/auth.c
> @@ -122,6 +122,8 @@ static int calc_ntlmv2_hash(struct ksmbd_conn *conn, struct ksmbd_session *sess,
>   out:
>   	kfree(uniname);
>   	kfree(domain);
> +	if (ret)	/* Done by hmac_md5_final() already if ret == 0 */
> +		memzero_explicit(&ctx, sizeof(ctx));
>   	return ret;
>   }
>   
> @@ -464,7 +466,7 @@ int ksmbd_krb5_authenticate(struct ksmbd_session *sess, char *in_blob,
>   	*out_len = resp->spnego_blob_len;
>   	retval = 0;
>   out:
> -	kvfree(resp);
> +	kvfree_sensitive(resp);
D'oh, self-nack, I really should not send patches in a hurry on Friday 
afternoon, sorry!

I will send a fixed version next week.

  Thomas