Re: [PATCH net-next v2 3/3] sctp: Convert cookie authentication to use HMAC-SHA256

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Aug 15, 2025 at 05:19:27PM -0400, Xin Long wrote:
> On Fri, Aug 15, 2025 at 3:09 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Tue, 12 Aug 2025 21:01:21 -0700 Eric Biggers wrote:
> > > +     if (net->sctp.cookie_auth_enable)
> > > +             tbl.data = (char *)"sha256";
> > > +     else
> > > +             tbl.data = (char *)"none";
> > > +     tbl.maxlen = strlen(tbl.data);
> > > +     return proc_dostring(&tbl, 0, buffer, lenp, ppos);
> >
> > I wonder if someone out there expects to read back what they wrote,
> > but let us find out.
> I feel it's a bit weird to have:
> 
> # sysctl net.sctp.cookie_hmac_alg="md5"
> net.sctp.cookie_hmac_alg = md5
> # sysctl net.sctp.cookie_hmac_alg
> net.sctp.cookie_hmac_alg = sha256
> 
> This patch deprecates md5 and sha1 use there.
> So generally, for situations like this, should we also issue a
> warning, or just fail it?
> 
> Paolo, what do you think?
> 
> >
> > It'd be great to get an ack / review from SCTP maintainers, otherwise
> > we'll apply by Monday..
> Other than that, LGTM.
> Sorry for the late reply, I was running some SCTP-auth related tests
> against the patchset.

Ideally we'd just fail the write and remove the last mentions of md5 and
sha1 from the code.  But I'm concerned there could be a case where
userspace is enabling cookie authentication by setting
cookie_hmac_alg=md5 or cookie_hmac_alg=sha1, and by just failing the
write the system would end up with cookie authentication not enabled.

It would have been nice if this sysctl had just been a boolean toggle.

A deprecation warning might be a good idea.  How about the following on
top of this patch:

diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
index 19acc57c3ed97..72af4a843ae52 100644
--- a/net/sctp/sysctl.c
+++ b/net/sctp/sysctl.c
@@ -399,20 +399,28 @@ static int proc_sctp_do_hmac_alg(const struct ctl_table *ctl, int write,
 		tbl.data = tmp;
 		tbl.maxlen = sizeof(tmp) - 1;
 		ret = proc_dostring(&tbl, 1, buffer, lenp, ppos);
 		if (ret)
 			return ret;
-		if (!strcmp(tmp, "sha256") ||
-		    /* for backwards compatibility */
-		    !strcmp(tmp, "md5") || !strcmp(tmp, "sha1")) {
+		if (!strcmp(tmp, "sha256")) {
 			net->sctp.cookie_auth_enable = 1;
 			return 0;
 		}
 		if (!strcmp(tmp, "none")) {
 			net->sctp.cookie_auth_enable = 0;
 			return 0;
 		}
+		/*
+		 * Accept md5 and sha1 for backwards compatibility, but treat
+		 * them simply as requests to enable cookie authentication.
+		 */
+		if (!strcmp(tmp, "md5") || !strcmp(tmp, "sha1")) {
+			pr_warn_once("net.sctp.cookie_hmac_alg=%s is deprecated. Use net.sctp.cookie_hmac_alg=sha256\n",
+				     tmp);
+			net->sctp.cookie_auth_enable = 1;
+			return 0;
+		}
 		return -EINVAL;
 	}
 	if (net->sctp.cookie_auth_enable)
 		tbl.data = (char *)"sha256";
 	else