Alice Mikityanska wrote: > On Thu, Aug 13, 2026, at 23:25, Willem de Bruijn wrote: > > Alice Mikityanska wrote: > >> From: Alice Mikityanska <[email protected]> > >> > >> Commit 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward") > >> dropped the IP6_MAX_MTU clamp that used to be present in ip6_mtu(). A > >> similar IPv4 commit ac6627a28dbf ("net: ipv4: Consolidate ipv4_mtu and > >> ip_dst_mtu_maybe_forward") preserves the IP_MAX_MTU clamp. > >> > >> Restore the upper bound in the IPv6 flow to avoid potential 16-bit > >> overflows in forwarding paths. > >> > >> Fixes: 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward") > >> Signed-off-by: Alice Mikityanska <[email protected]> > >> Suggested-by: Willem de Bruijn <[email protected]> > > > > Reviewed-by: Willem de Bruijn <[email protected]> > > > >> --- > >> include/net/ip6_route.h | 2 ++ > >> 1 file changed, 2 insertions(+) > >> > >> diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h > >> index 09ffe0f13ce7..fb59a5885faa 100644 > >> --- a/include/net/ip6_route.h > >> +++ b/include/net/ip6_route.h > >> @@ -382,6 +382,8 @@ static inline unsigned int > >> ip6_dst_mtu_maybe_forward(const struct dst_entry *dst > >> rcu_read_unlock(); > >> > >> out: > >> + mtu = min_t(unsigned int, mtu, IP6_MAX_MTU); > >> + > >> return mtu - lwtunnel_headroom(dst->lwtstate, mtu); > >> } > > > > It appears IPv4 only clamps device MTU, not route MTU: > > > > mtu = dst_metric_raw(dst, RTAX_MTU); > > if (!mtu) > > mtu = min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU); > > This is some old code from v5.14, it changed in commit ac6627a28dbf > ("net: ipv4: Consolidate ipv4_mtu and ip_dst_mtu_maybe_forward"), and > IPv4 clamps MTU in both cases since then.
Oh indeed. > > I don't think that was necessarily intentional. Perhaps route MTU > > itself is already bounds checked on configuration. The device MTU > > min() was added after a syzbot report, in commit c780a049f9b. > > This commit merely adds READ_ONCE to the existing min. > > > Current IPv6 proposal is arguably more robust, covering both. There > > just remains a difference between IPv4 and IPv6 code paths. > > So, looking at the fresh checkout, it seems that my patch covers the > difference, right? Yes, agreed.
