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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: Phil Sutter , Simon Horman , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Xiang Mei , Weiming Shi Subject: [PATCH nf] netfilter: nf_tables: use RCU-safe list primitives for basechain hook list Date: Fri, 10 Apr 2026 18:13:22 +0800 Message-ID: <20260410101321.915190-2-bestswngs@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" NFT_MSG_GETCHAIN runs as an NFNL_CB_RCU callback, so chain dumps traverse basechain->hook_list under rcu_read_lock() without holding commit_mutex. Meanwhile, nft_delchain_hook() mutates that same live hook_list with plain list_move() and list_splice(), and the commit/abort paths splice hooks back with plain list_splice(). None of these are RCU-safe list operations. A concurrent GETCHAIN dump can observe partially updated list pointers, follow them into stack-local or transaction-private list heads, and crash when container_of() produces a bogus struct nft_hook pointer. The PoC triggers this by racing GETCHAIN dumps against aborting DELCHAIN hook updates, reachable from an unprivileged user namespace since all capability checks use ns_capable() with CONFIG_NF_TABLES=3Dy (default): Oops: general protection fault, probably for non-canonical address 0xdffff= c0000000006: 0000 [#1] SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037] RIP: 0010:strlen (lib/string.c:420 (discriminator 1)) Call Trace: nf_tables_fill_chain_info (net/netfilter/nf_tables_api.c:1987 (discrimina= tor 1) net/netfilter/nf_tables_api.c:1992 (discriminator 1) net/netfilter/n= f_tables_api.c:2028 (discriminator 1) net/netfilter/nf_tables_api.c:2077 (d= iscriminator 1)) nf_tables_dump_chains (net/netfilter/nf_tables_api.c:2173 (discriminator = 1)) netlink_dump (net/netlink/af_netlink.c:2325 (discriminator 1)) __netlink_dump_start (net/netlink/af_netlink.c:2442) nf_tables_getchain (net/netfilter/nf_tables_api.c:1314 net/netfilter/nf_t= ables_api.c:2212) nfnetlink_rcv_msg (net/netfilter/nfnetlink.c:290) netlink_rcv_skb (net/netlink/af_netlink.c:2550) nfnetlink_rcv (net/netfilter/nfnetlink.c:653) netlink_unicast (net/netlink/af_netlink.c:1319 net/netlink/af_netlink.c:1= 344) netlink_sendmsg (net/netlink/af_netlink.c:1894) __sys_sendto (net/socket.c:727 net/socket.c:742 net/socket.c:2206) __x64_sys_sendto (net/socket.c:2209) Replace list_move() in nft_delchain_hook() with list_del_rcu() plus an intermediate pointer array, followed by synchronize_rcu() before the deleted hooks' list pointers are reused to link them into the transaction's private list. In the error paths, put hooks back with list_add_tail_rcu() which is safe for concurrent RCU readers (they either continue to the original successor or see the list head and terminate the walk). Add nft_hook_list_splice_rcu() helper that splices entries from a private list into a live RCU-protected list using individual list_add_tail_rcu() calls instead of plain list_splice(). Use it in the commit and abort paths for NEWCHAIN updates and DELCHAIN rollback. Fixes: 7d937b107108 ("netfilter: nf_tables: support for deleting devices in= an existing netdev chain") Reported-by: Xiang Mei Signed-off-by: Weiming Shi --- net/netfilter/nf_tables_api.c | 64 ++++++++++++++++++++++++++++++----- 1 file changed, 56 insertions(+), 8 deletions(-) diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c index 8c42247a176c7..62fcfefba7b0f 100644 --- a/net/netfilter/nf_tables_api.c +++ b/net/netfilter/nf_tables_api.c @@ -391,6 +391,22 @@ static void nft_netdev_unregister_hooks(struct net *ne= t, } } =20 +/* Splice hooks from a private list into a live (RCU-protected) hook list. + * Each entry is published individually via list_add_tail_rcu() so that + * concurrent RCU readers walking the destination list never observe torn + * list pointers. + */ +static void nft_hook_list_splice_rcu(struct list_head *from, + struct list_head *to) +{ + struct nft_hook *hook, *next; + + list_for_each_entry_safe(hook, next, from, list) { + list_del(&hook->list); + list_add_tail_rcu(&hook->list, to); + } +} + static int nf_tables_register_hook(struct net *net, const struct nft_table *table, struct nft_chain *chain) @@ -3162,9 +3178,11 @@ static int nft_delchain_hook(struct nft_ctx *ctx, const struct nlattr * const *nla =3D ctx->nla; struct nft_chain_hook chain_hook =3D {}; struct nft_hook *this, *hook; + struct nft_hook **del_hooks; LIST_HEAD(chain_del_list); struct nft_trans *trans; - int err; + int err, n =3D 0, i; + int max_hooks =3D 0; =20 if (ctx->table->flags & __NFT_TABLE_F_UPDATE) return -EOPNOTSUPP; @@ -3174,19 +3192,38 @@ static int nft_delchain_hook(struct nft_ctx *ctx, if (err < 0) return err; =20 + list_for_each_entry(this, &chain_hook.list, list) + max_hooks++; + + del_hooks =3D kcalloc(max_hooks, sizeof(*del_hooks), GFP_KERNEL); + if (!del_hooks) { + nft_chain_release_hook(&chain_hook); + return -ENOMEM; + } + list_for_each_entry(this, &chain_hook.list, list) { hook =3D nft_hook_list_find(&basechain->hook_list, this); if (!hook) { err =3D -ENOENT; goto err_chain_del_hook; } - list_move(&hook->list, &chain_del_list); + list_del_rcu(&hook->list); + del_hooks[n++] =3D hook; } =20 + /* Wait for any concurrent RCU readers (e.g. GETCHAIN dumps walking + * basechain->hook_list) to finish before modifying the removed hooks' + * list pointers to link them into the transaction's private list. + */ + synchronize_rcu(); + + for (i =3D 0; i < n; i++) + list_add_tail(&del_hooks[i]->list, &chain_del_list); + trans =3D nft_trans_alloc_chain(ctx, NFT_MSG_DELCHAIN); if (!trans) { err =3D -ENOMEM; - goto err_chain_del_hook; + goto err_chain_add_back; } =20 nft_trans_basechain(trans) =3D basechain; @@ -3194,13 +3231,24 @@ static int nft_delchain_hook(struct nft_ctx *ctx, INIT_LIST_HEAD(&nft_trans_chain_hooks(trans)); list_splice(&chain_del_list, &nft_trans_chain_hooks(trans)); nft_chain_release_hook(&chain_hook); + kfree(del_hooks); =20 nft_trans_commit_list_add_tail(ctx->net, trans); =20 return 0; =20 +err_chain_add_back: + for (i =3D 0; i < n; i++) + list_add_tail_rcu(&del_hooks[i]->list, &basechain->hook_list); + kfree(del_hooks); + nft_chain_release_hook(&chain_hook); + + return err; + err_chain_del_hook: - list_splice(&chain_del_list, &basechain->hook_list); + for (i =3D 0; i < n; i++) + list_add_tail_rcu(&del_hooks[i]->list, &basechain->hook_list); + kfree(del_hooks); nft_chain_release_hook(&chain_hook); =20 return err; @@ -10912,8 +10960,8 @@ static int nf_tables_commit(struct net *net, struct= sk_buff *skb) nft_chain_commit_update(nft_trans_container_chain(trans)); nf_tables_chain_notify(&ctx, NFT_MSG_NEWCHAIN, &nft_trans_chain_hooks(trans)); - list_splice(&nft_trans_chain_hooks(trans), - &nft_trans_basechain(trans)->hook_list); + nft_hook_list_splice_rcu(&nft_trans_chain_hooks(trans), + &nft_trans_basechain(trans)->hook_list); /* trans destroyed after rcu grace period */ } else { nft_chain_commit_drop_policy(nft_trans_container_chain(trans)); @@ -11231,8 +11279,8 @@ static int __nf_tables_abort(struct net *net, enum = nfnl_abort_action action) case NFT_MSG_DELCHAIN: case NFT_MSG_DESTROYCHAIN: if (nft_trans_chain_update(trans)) { - list_splice(&nft_trans_chain_hooks(trans), - &nft_trans_basechain(trans)->hook_list); + nft_hook_list_splice_rcu(&nft_trans_chain_hooks(trans), + &nft_trans_basechain(trans)->hook_list); } else { nft_use_inc_restore(&table->use); nft_clear(trans->net, nft_trans_chain(trans)); --=20 2.43.0