netdev - [PATCH] net: Only allow init netns to set default tcp cong to a restricted algo

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Date:   Sat,  1 May 2021 04:28:22 -0400
From:   Jonathon Reinhart <jonathon.reinhart@...il.com>
To:     netdev@...r.kernel.org
Cc:     Jonathon Reinhart <jonathon.reinhart@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: [PATCH] net: Only allow init netns to set default tcp cong to a restricted algo

tcp_set_default_congestion_control() is netns-safe in that it writes
to &net->ipv4.tcp_congestion_control, but it also sets
ca->flags |= TCP_CONG_NON_RESTRICTED which is not namespaced.
This has the unintended side-effect of changing the global
net.ipv4.tcp_allowed_congestion_control sysctl, despite the fact that it
is read-only: 97684f0970f6 ("net: Make tcp_allowed_congestion_control
readonly in non-init netns")

Resolve this netns "leak" by only allowing the init netns to set the
default algorithm to one that is restricted. This restriction could be
removed if tcp_allowed_congestion_control were namespace-ified in the
future.

This bug was uncovered with
https://github.com/JonathonReinhart/linux-netns-sysctl-verify

Fixes: 6670e1524477 ("tcp: Namespace-ify sysctl_tcp_default_congestion_control")
Signed-off-by: Jonathon Reinhart <jonathon.reinhart@...il.com>
---
 net/ipv4/tcp_cong.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 563d016e7478..db5831e6c136 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -230,6 +230,10 @@ int tcp_set_default_congestion_control(struct net *net, const char *name)
 		ret = -ENOENT;
 	} else if (!bpf_try_module_get(ca, ca->owner)) {
 		ret = -EBUSY;
+	} else if (!net_eq(net, &init_net) &&
+			!(ca->flags & TCP_CONG_NON_RESTRICTED)) {
+		/* Only init netns can set default to a restricted algorithm */
+		ret = -EPERM;
 	} else {
 		prev = xchg(&net->ipv4.tcp_congestion_control, ca);
 		if (prev)
-- 
2.20.1