[PATCH 6.1 137/473] tcp: avoid premature drops in tcp_add_backlog()

Greg Kroah-Hartman
2024-06-06T14:14:21Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit ec00ed472bdb7d0af840da68c8c11bff9f4d9caa ]

While testing TCP performance with latest trees,
I saw suspect SOCKET_BACKLOG drops.

tcp_add_backlog() computes its limit with :

limit = (u32)READ_ONCE(sk->sk_rcvbuf) +
(u32)(READ_ONCE(sk->sk_sndbuf) >> 1);
limit += 64 * 1024;

This does not take into account that sk->sk_backlog.len
is reset only at the very end of __release_sock().

Both sk->sk_backlog.len and sk->sk_rmem_alloc could reach
sk_rcvbuf in normal conditions.

We should double sk->sk_rcvbuf contribution in the formula
to absorb bubbles in the backlog, which happen more often
for very fast flows.

This change maintains decent protection against abuses.

Fixes: c377411f2494 ("net: sk_add_backlog() take rmem_alloc into account")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240423125620.3309458-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv4/tcp_ipv4.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 5dcb969cb5e9c..befa848fb820c 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1771,7 +1771,7 @@ int tcp_v4_early_demux(struct sk_buff *skb)
bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
enum skb_drop_reason *reason)
{
- u32 limit, tail_gso_size, tail_gso_segs;
+ u32 tail_gso_size, tail_gso_segs;
struct skb_shared_info *shinfo;
const struct tcphdr *th;
struct tcphdr *thtail;
@@ -1780,6 +1780,7 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
bool fragstolen;
u32 gso_segs;
u32 gso_size;
+ u64 limit;
int delta;

/* In case all data was pulled from skb frags (in __pskb_pull_tail()),
@@ -1877,7 +1878,13 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
__skb_push(skb, hdrlen);

no_coalesce:
- limit = (u32)READ_ONCE(sk->sk_rcvbuf) + (u32)(READ_ONCE(sk->sk_sndbuf) >> 1);
+ /* sk->sk_backlog.len is reset only at the end of __release_sock().
+ * Both sk->sk_backlog.len and sk->sk_rmem_alloc could reach
+ * sk_rcvbuf in normal conditions.
+ */
+ limit = ((u64)READ_ONCE(sk->sk_rcvbuf)) << 1;
+
+ limit += ((u32)READ_ONCE(sk->sk_sndbuf)) >> 1;

/* Only socket owner can try to collapse/prune rx queues
* to reduce memory overhead, so add a little headroom here.
@@ -1885,6 +1892,8 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
*/
limit += 64 * 1024;

+ limit = min_t(u64, limit, UINT_MAX);
+
if (unlikely(sk_add_backlog(sk, skb, limit))) {
bh_unlock_sock(sk);
*reason = SKB_DROP_REASON_SOCKET_BACKLOG;
--
2.43.0