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Subject[PATCH 5.10 527/530] net: Only allow init netns to set default tcp cong to a restricted algo
From: Jonathon Reinhart <>

commit 8d432592f30fcc34ef5a10aac4887b4897884493 upstream.

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

This bug was uncovered with

Fixes: 6670e1524477 ("tcp: Namespace-ify sysctl_tcp_default_congestion_control")
Signed-off-by: Jonathon Reinhart <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/ipv4/tcp_cong.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -230,6 +230,10 @@ int tcp_set_default_congestion_control(s
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)

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