[PATCH] netfilter: nft_flow_offload: drop flowtable reference on init error path

[Aohan Mei <ljp1205831794@gmail.com>]

From: Aohan Mei <henrymei@tencent.com>

nft_flow_offload_init() bumps the flowtable use count with
nft_use_inc() before calling nf_ct_netns_get().  When the latter
fails, the error is returned as-is and the reference is leaked.

The upper layers do not balance it either: nf_tables_newexpr()
clears expr->ops when the expression init callback fails, so the
nft_expr_more() iteration in nft_rule_expr_deactivate() and
nf_tables_rule_destroy() stops right before the failed expression
and its ->destroy callback, which would drop the reference, never
runs.

Each failed rule addition therefore leaks one flowtable reference
and the flowtable can no longer be removed: NFT_MSG_DELFLOWTABLE
keeps reporting -EBUSY even though no rule references it.

Save the nf_ct_netns_get() return value and undo the nft_use_inc()
when it fails, restoring the inc/dec pairing within
nft_flow_offload_init() itself.

Fixes: a3c90f7a2323 ("netfilter: nf_tables: flow offload expression")
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Cc: stable@vger.kernel.org
Assisted-by: CodeBuddy:Kimi-K3
Signed-off-by: Aohan Mei <henrymei@tencent.com>
---
 net/netfilter/nft_flow_offload.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nft_flow_offload.c b/net/netfilter/nft_flow_offload.c
index 32b4281038dd..d3c5651dd699 100644
--- a/net/netfilter/nft_flow_offload.c
+++ b/net/netfilter/nft_flow_offload.c
@@ -160,6 +160,7 @@ static int nft_flow_offload_init(const struct nft_ctx *ctx,
 	struct nft_flow_offload *priv = nft_expr_priv(expr);
 	u8 genmask = nft_genmask_next(ctx->net);
 	struct nft_flowtable *flowtable;
+	int err;
 
 	if (!tb[NFTA_FLOW_TABLE_NAME])
 		return -EINVAL;
@@ -174,7 +175,11 @@ static int nft_flow_offload_init(const struct nft_ctx *ctx,
 
 	priv->flowtable = flowtable;
 
-	return nf_ct_netns_get(ctx->net, ctx->family);
+	err = nf_ct_netns_get(ctx->net, ctx->family);
+	if (err < 0)
+		nft_use_dec(&flowtable->use);
+
+	return err;
 }
 
 static void nft_flow_offload_deactivate(const struct nft_ctx *ctx,
-- 
2.43.7