On 6/10/22 2:17 AM, Martin KaFai Lau wrote:
On Thu, Jun 09, 2022 at 10:29:15PM +0200, Daniel Borkmann wrote:
On 6/9/22 3:18 AM, Jon Maxwell wrote:
A customer reported a request_socket leak in a Calico cloud environment. We
found that a BPF program was doing a socket lookup with takes a refcnt on
the socket and that it was finding the request_socket but returning the parent
LISTEN socket via sk_to_full_sk() without decrementing the child request socket
1st, resulting in request_sock slab object leak. This patch retains the
Great catch and debug indeed!
existing behaviour of returning full socks to the caller but it also decrements
the child request_socket if one is present before doing so to prevent the leak.
Thanks to Curtis Taylor for all the help in diagnosing and testing this. And
thanks to Antoine Tenart for the reproducer and patch input.
Fixes: f7355a6c0497 bpf: ("Check sk_fullsock() before returning from bpf_sk_lookup()")
Fixes: edbf8c01de5a bpf: ("add skc_lookup_tcp helper")
Instead of the above commits, I think this dated back to
6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
Tested-by: Curtis Taylor <cutaylor-pub@xxxxxxxxx>
Co-developed-by: Antoine Tenart <atenart@xxxxxxxxxx>
Signed-off-by:: Antoine Tenart <atenart@xxxxxxxxxx>
Signed-off-by: Jon Maxwell <jmaxwell37@xxxxxxxxx>
---
net/core/filter.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)
diff --git a/net/core/filter.c b/net/core/filter.c
index 2e32cee2c469..e3c04ae7381f 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6202,13 +6202,17 @@ __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
{
struct sock *sk = __bpf_skc_lookup(skb, tuple, len, caller_net,
ifindex, proto, netns_id, flags);
+ struct sock *sk1 = sk;
if (sk) {
sk = sk_to_full_sk(sk);
- if (!sk_fullsock(sk)) {
- sock_gen_put(sk);
+ /* sk_to_full_sk() may return (sk)->rsk_listener, so make sure the original sk1
+ * sock refcnt is decremented to prevent a request_sock leak.
+ */
+ if (!sk_fullsock(sk1))
+ sock_gen_put(sk1);
+ if (!sk_fullsock(sk))
In this case, sk1 == sk (timewait). It is a bit worrying to pass
sk to sk_fullsock(sk) after the above sock_gen_put().
I think Daniel's 'if (sk2 != sk) { sock_gen_put(sk); }' check is better.
[ +Martin/Joe/Lorenz ]
I wonder, should we also add some asserts in here to ensure we don't get an unbalance for the
bpf_sk_release() case later on? Rough pseudocode could be something like below:
static struct sock *
__bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
struct net *caller_net, u32 ifindex, u8 proto, u64 netns_id,
u64 flags)
{
struct sock *sk = __bpf_skc_lookup(skb, tuple, len, caller_net,
ifindex, proto, netns_id, flags);
if (sk) {
struct sock *sk2 = sk_to_full_sk(sk);
if (!sk_fullsock(sk2))
sk2 = NULL;
if (sk2 != sk) {
sock_gen_put(sk);
if (unlikely(sk2 && !sock_flag(sk2, SOCK_RCU_FREE))) {
I don't think it matters if the helper-returned sk2 is refcounted or not (SOCK_RCU_FREE).
The verifier has ensured the bpf_sk_lookup() and bpf_sk_release() are
always balanced regardless of the type of sk2.
bpf_sk_release() will do the right thing to check the sk2 is refcounted or not
before calling sock_gen_put().
The bug here is the helper forgot to call sock_gen_put(sk) while
the verifier only tracks the sk2, so I think the 'if (unlikely...) { WARN_ONCE(...); }'
can be saved.
I was mainly thinking given in sk_lookup() we have the check around `sk && !refcounted &&
!sock_flag(sk, SOCK_RCU_FREE)` to check for unreferenced non-SOCK_RCU_FREE socket, and
given sk_to_full_sk() can return inet_reqsk(sk)->rsk_listener we don't have a similar
assertion there. Given we don't bump any ref on the latter, it must be SOCK_RCU_FREE then
as otherwise latter call to bpf_sk_release() will unbalance sk2. @Jon: maybe lets just
manually verify that such sk2 has SOCK_RCU_FREE and state it in the commit message for
future reference then, either is fine with me. Thanks!
WARN_ONCE(1, "Found non-RCU, unreferenced socket!");
sk2 = NULL;
}
}
sk = sk2;
}
return sk;
}
