netfilter: nf_tables: fix use-after-free when deleting compat expressions

nft_compat ops do not have static storage duration, unlike all other
expressions.

When nf_tables_expr_destroy() returns, expr->ops might have been
free'd already, so we need to store next address before calling
expression destructor.

For same reason, we can't deref match pointer after nft_xt_put().

This can be easily reproduced by adding msleep() before
nft_match_destroy() returns.

Fixes: 0ca743a55991 ("netfilter: nf_tables: add compatibility layer for x_tables")
Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -2457,7 +2457,7 @@ err:
 static void nf_tables_rule_destroy(const struct nft_ctx *ctx,
 				   struct nft_rule *rule)
 {
-	struct nft_expr *expr;
+	struct nft_expr *expr, *next;
 
 	/*
 	 * Careful: some expressions might not be initialized in case this
@@ -2465,8 +2465,9 @@ static void nf_tables_rule_destroy(const struct nft_ctx *ctx,
 	 */
 	expr = nft_expr_first(rule);
 	while (expr != nft_expr_last(rule) && expr->ops) {
+		next = nft_expr_next(expr);
 		nf_tables_expr_destroy(ctx, expr);
-		expr = nft_expr_next(expr);
+		expr = next;
 	}
 	kfree(rule);
 }

net/netfilter/nft_compat.c

@@ -520,6 +520,7 @@ __nft_match_destroy(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		    void *info)
 {
 	struct xt_match *match = expr->ops->data;
+	struct module *me = match->me;
 	struct xt_mtdtor_param par;
 
 	par.net = ctx->net;
@@ -530,7 +531,7 @@ __nft_match_destroy(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		par.match->destroy(&par);
 
 	if (nft_xt_put(container_of(expr->ops, struct nft_xt, ops)))
-		module_put(match->me);
+		module_put(me);
 }
 
 static void