[PATCH] ipv6/addrconf: clamp preferred_lft to the minimum instead of erroring

[Alex Henrie <alexhenrie24@gmail.com>]

I tried setting /proc/sys/net/ipv6/conf/*/temp_prefered_lft to 1 so that
the address would roll over as frequently as possible, then spent hours
trying to understand why the preferred lifetime jumped to 4 billion
seconds. On my machine and network the shortest lifetime that avoids
underflow is 3 seconds.

After fixing the underflow, I ran into a second problem: The preferred
lifetime was less than the minimum required lifetime, so
ipv6_create_tempaddr would error out without creating any new address.
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/1031168/310447
Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
---
 net/ipv6/addrconf.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 94cec2075eee..4008d4a5e58d 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -1368,7 +1368,7 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, bool block)
 	 * idev->desync_factor if it's larger
 	 */
 	cnf_temp_preferred_lft = READ_ONCE(idev->cnf.temp_prefered_lft);
-	max_desync_factor = min_t(__u32,
+	max_desync_factor = min_t(__s64,
 				  idev->cnf.max_desync_factor,
 				  cnf_temp_preferred_lft - regen_advance);
 
@@ -1402,12 +1402,8 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, bool block)
 	 * temporary addresses being generated.
 	 */
 	age = (now - tmp_tstamp + ADDRCONF_TIMER_FUZZ_MINUS) / HZ;
-	if (cfg.preferred_lft <= regen_advance + age) {
-		in6_ifa_put(ifp);
-		in6_dev_put(idev);
-		ret = -1;
-		goto out;
-	}
+	if (cfg.preferred_lft <= regen_advance + age)
+		cfg.preferred_lft = regen_advance + age + 1;
 
 	cfg.ifa_flags = IFA_F_TEMPORARY;
 	/* set in addrconf_prefix_rcv() */
-- 
2.41.0