[iptables PATCH 1/4] nft: cache: Fix nft_release_cache() under stress

[Phil Sutter <phil@nwl.cc>]

iptables-nft-restore calls nft_action(h, NFT_COMPAT_COMMIT) for each
COMMIT line in input. When restoring a dump containing multiple large
tables, chances are nft_rebuild_cache() has to run multiple times.

If the above happens, consecutive table contents are added to __cache[1]
which nft_rebuild_cache() then frees, so next commit attempt accesses
invalid memory.

Fix this by making nft_release_cache() (called after each successful
commit) return things into pre-rebuild state again, but keeping the
fresh cache copy.

Fixes: f6ad231d698c7 ("nft: keep original cache in case of ERESTART")
Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 iptables/nft-cache.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/iptables/nft-cache.c b/iptables/nft-cache.c
index 7345a27e2894b..6f21f2283e0fb 100644
--- a/iptables/nft-cache.c
+++ b/iptables/nft-cache.c
@@ -647,8 +647,14 @@ void nft_rebuild_cache(struct nft_handle *h)
 
 void nft_release_cache(struct nft_handle *h)
 {
-	if (h->cache_index)
-		flush_cache(h, &h->__cache[0], NULL);
+	if (!h->cache_index)
+		return;
+
+	flush_cache(h, &h->__cache[0], NULL);
+	memcpy(&h->__cache[0], &h->__cache[1], sizeof(h->__cache[0]));
+	memset(&h->__cache[1], 0, sizeof(h->__cache[1]));
+	h->cache_index = 0;
+	h->cache = &h->__cache[0];
 }
 
 struct nftnl_table_list *nftnl_table_list_get(struct nft_handle *h)
-- 
2.25.1