On Fri, Nov 4, 2022 at 10:31 AM Pavel Machek pavel@denx.de wrote:
Hi!
[ Upstream commit d89d7ff01235f218dad37de84457717f699dee79 ]
Another syzbot report [1] with no reproducer hints at a bug in ip6_gre tunnel (dev:ip6gretap0)
Since ipv6 mcast code makes sure to read dev->mtu once and applies a sanity check on it (see commit b9b312a7a451 "ipv6: mcast: better catch silly mtu values"), a remaining possibility is that a layer is able to set dev->mtu to an underflowed value (high order bit set).
This could happen indeed in ip6gre_tnl_link_config_route(), ip6_tnl_link_config() and ipip6_tunnel_bind_dev()
Make sure to sanitize mtu value in a local variable before it is written once on dev->mtu, as lockless readers could catch wrong temporary value.
Ok, but now types seem to be confused:
diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c index 3a2741569b84..0d4cab94c5dd 100644 --- a/net/ipv6/ip6_tunnel.c +++ b/net/ipv6/ip6_tunnel.c @@ -1476,8 +1476,8 @@ static void ip6_tnl_link_config(struct ip6_tnl *t) struct net_device *tdev = NULL; struct __ip6_tnl_parm *p = &t->parms; struct flowi6 *fl6 = &t->fl.u.ip6;
unsigned int mtu; int t_hlen;
int mtu; memcpy(dev->dev_addr, &p->laddr, sizeof(struct in6_addr)); memcpy(dev->broadcast, &p->raddr, sizeof(struct in6_addr));@@ -1524,12 +1524,13 @@ static void ip6_tnl_link_config(struct ip6_tnl *t) dev->hard_header_len = tdev->hard_header_len + t_hlen; mtu = min_t(unsigned int, tdev->mtu, IP6_MAX_MTU);
mtu is now signed, but we still do min_t on unsigned types.
dev->mtu = mtu - t_hlen;
mtu = mtu - t_hlen; if (!(t->parms.flags & IP6_TNL_F_IGN_ENCAP_LIMIT))
dev->mtu -= 8;
mtu -= 8;I don't see overflow potential right away, but it may be worth fixing.
This was intended ( part of the fix) so that the following check is going to catch 'negative' mtu
[1] if (mtu < IPV6_MIN_MTU) mtu = IPV6_MIN_MTU;
Otherwise, if a fuzzer succeeds to get mtu = 0xFFFFFFC0, sanity test [1] leaves the problematic mtu in dev->mtu.
This is the line I'm complaining about (1525 in 5.10):
mtu = min_t(unsigned int, tdev->mtu, IP6_MAX_MTU);
I don't think it does any harm, but it looks wrong/confusing.
So you are confused by :
some_integer_var = some_unsigned_int_expression;
I do not see any issue with that.
Thanks.