net: ipv6/addrconf: clamp preferred_lft to the minimum required

If the preferred lifetime was less than the minimum required lifetime,
ipv6_create_tempaddr would error out without creating any new address.
On my machine and network, this error happened immediately with the
preferred lifetime set to 1 second, after a few minutes with the
preferred lifetime set to 4 seconds, and not at all with the preferred
lifetime set to 5 seconds. During my investigation, I found a Stack
Exchange post from another person who seems to have had the same
problem: They stopped getting new addresses if they lowered the
preferred lifetime below 3 seconds, and they didn't really know why.

The preferred lifetime is a preference, not a hard requirement. The
kernel does not strictly forbid new connections on a deprecated address,
nor does it guarantee that the address will be disposed of the instant
its total valid lifetime expires. So rather than disable IPv6 privacy
extensions altogether if the minimum required lifetime swells above the
preferred lifetime, it is more in keeping with the user's intent to
increase the temporary address's lifetime to the minimum necessary for
the current network conditions.

With these fixes, setting the preferred lifetime to 3 or 4 seconds "just
works" because the extra fraction of a second is practically
unnoticeable. It's even possible to reduce the time before deprecation
to 1 or 2 seconds by also disabling duplicate address detection (setting
/proc/sys/net/ipv6/conf/*/dad_transmits to 0). I realize that that is a
pretty niche use case, but I know at least one person who would gladly
sacrifice performance and convenience to be sure that they are getting
the maximum possible level of privacy.

Link: https://serverfault.com/a/1031168/310447Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231024212312.299370-3-alexhenrie24@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/addrconf.c

@@ -1407,15 +1407,23 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, bool block)
 
 	write_unlock_bh(&idev->lock);
 
-	/* A temporary address is created only if this calculated Preferred
-	 * Lifetime is greater than REGEN_ADVANCE time units.  In particular,
-	 * an implementation must not create a temporary address with a zero
-	 * Preferred Lifetime.
+	/* From RFC 4941:
+	 *
+	 *     A temporary address is created only if this calculated Preferred
+	 *     Lifetime is greater than REGEN_ADVANCE time units.  In
+	 *     particular, an implementation must not create a temporary address
+	 *     with a zero Preferred Lifetime.
+	 *
+	 * Clamp the preferred lifetime to a minimum of regen_advance, unless
+	 * that would exceed valid_lft.
+	 *
 	 * Use age calculation as in addrconf_verify to avoid unnecessary
 	 * temporary addresses being generated.
 	 */
 	age = (now - tmp_tstamp + ADDRCONF_TIMER_FUZZ_MINUS) / HZ;
-	if (cfg.preferred_lft <= regen_advance + age) {
+	if (cfg.preferred_lft <= regen_advance + age)
+		cfg.preferred_lft = regen_advance + age + 1;
+	if (cfg.preferred_lft > cfg.valid_lft) {
 		in6_ifa_put(ifp);
 		in6_dev_put(idev);
 		ret = -1;