Re: [PATCH v5 12/15] tcp, udp: Bind outbound listening sockets by interface instead of address

Sorry, I realised the warning I add here isn't correct in a couple of ways. I'll respin shortly, but patches 1..11 should still be fine to review. On Tue, Dec 02, 2025 at 03:02:12PM +1100, David Gibson wrote:

Currently, outbound forwards (-T, -U) are handled by sockets bound to the loopback address. Typically we create two sockets, one for 127.0.0.1 and one for ::1.

This has some disadvantages: * The guest can't connect via 127.0.0.0/8 addresses other than 127.0.0.1 * We can't use dual-stack sockets, we have to have separate sockets for IPv4 and IPv6.

The restriction exists for a reason though. If the guest has any interfaces other than pasta (e.g. a VPN tunnel) external hosts could reach the host via the forwards. Especially combined with -T auto / -U auto this would make it very easy to make a mistake with nasty security implications.

We can achieve this a different way, however. Don't bind to a specific address, but _do_ use SO_BINDTODEVICE to restrict the sockets to the "lo" interface. We fall back to the old behaviour for older kernels where SO_BINDTODEVICE is not available unprivileged.

Note that although traffic to a local but non-loopback address is passed over the 'lo' interface (as seen by netfilter and dumpcap), it doesn't count as attached to that interface for the purposes of SO_BINDTODEVICE (information from the routing table overrides the "physical" interface). So, this change doesn't help for bug 100.

It's also not a complete fix for bug 113, it does however: * Get us a step closer to fixing bug 113 * Slightly simplify the code * Make things a bit easier to allow more flexible binding on the guest in in future

Link: https://bugs.passt.top/show_bug.cgi?id=113

Signed-off-by: David Gibson --- conf.c | 6 ++++++ pif.c | 6 ------ tcp.c | 5 +++++ udp.c | 30 +++++++++++++++++++++++------- 4 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/conf.c b/conf.c index 6bd9717b..02a4b65a 100644 --- a/conf.c +++ b/conf.c @@ -235,6 +235,12 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg, if (c->mode != MODE_PASTA) die("'auto' port forwarding is only allowed for pasta");

+ if ((optname == 'T' || optname == 'U') && c->no_bindtodevice) { + warn( +"'-%c auto' enabled without unprivileged SO_BINDTODEVICE", optname); + warn( +"Forwarding from addresses other than 127.0.0.1 will not work"); + } fwd->mode = FWD_AUTO; return; } diff --git a/pif.c b/pif.c index 85904f35..db447b4f 100644 --- a/pif.c +++ b/pif.c @@ -81,12 +81,6 @@ int pif_sock_l4(const struct ctx *c, enum epoll_type type, uint8_t pif,

ASSERT(pif_is_socket(pif));

- if (pif == PIF_SPLICE) { - /* Sanity checks */ - ASSERT(!ifname); - ASSERT(addr && inany_is_loopback(addr)); - } - if (!addr) { ref.fd = sock_l4_dualstack(c, type, port, ifname); } else { diff --git a/tcp.c b/tcp.c index 2abb8be4..aacc5b20 100644 --- a/tcp.c +++ b/tcp.c @@ -2627,6 +2627,11 @@ static void tcp_ns_sock_init(const struct ctx *c, in_port_t port) { ASSERT(!c->no_tcp);

+ if (!c->no_bindtodevice) { + tcp_sock_init(c, PIF_SPLICE, NULL, "lo", port); + return; + } + if (c->ifi4) tcp_sock_init_one(c, PIF_SPLICE, &inany_loopback4, NULL, port); if (c->ifi6) diff --git a/udp.c b/udp.c index 3d097fbb..4b625b78 100644 --- a/udp.c +++ b/udp.c @@ -1182,6 +1182,26 @@ static void udp_splice_iov_init(void) } }

+/** + * udp_ns_sock_init() - Init socket to listen for spliced outbound connections + * @c: Execution context + * @port: Port, host order + */ +static void udp_ns_sock_init(const struct ctx *c, in_port_t port) +{ + ASSERT(!c->no_udp); + + if (!c->no_bindtodevice) { + udp_sock_init(c, PIF_SPLICE, NULL, "lo", port); + return; + } + + if (c->ifi4) + udp_sock_init(c, PIF_SPLICE, &inany_loopback4, NULL, port); + if (c->ifi6) + udp_sock_init(c, PIF_SPLICE, &inany_loopback6, NULL, port); +} + /** * udp_port_rebind() - Rebind ports to match forward maps * @c: Execution context @@ -1213,14 +1233,10 @@ static void udp_port_rebind(struct ctx *c, bool outbound)

if ((c->ifi4 && socks[V4][port] == -1) || (c->ifi6 && socks[V6][port] == -1)) { - if (outbound) { - udp_sock_init(c, PIF_SPLICE, - &inany_loopback4, NULL, port); - udp_sock_init(c, PIF_SPLICE, - &inany_loopback6, NULL, port); - } else { + if (outbound) + udp_ns_sock_init(c, port); + else udp_sock_init(c, PIF_HOST, NULL, NULL, port); - } } } } -- 2.52.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson