From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
To: Fernando Fernandez Mancera <fmancera@suse.de>
Cc: <netdev@vger.kernel.org>, <csmate@nop.hu>,
<kerneljasonxing@gmail.com>, <bjorn@kernel.org>,
<sdf@fomichev.me>, <jonathan.lemon@gmail.com>,
<bpf@vger.kernel.org>, <davem@davemloft.net>,
<edumazet@google.com>, <kuba@kernel.org>, <pabeni@redhat.com>,
<horms@kernel.org>
Subject: Re: [PATCH net v3] xsk: avoid data corruption on cq descriptor number
Date: Mon, 17 Nov 2025 17:08:43 +0100 [thread overview]
Message-ID: <aRtIiIvfVwJCmcn1@boxer> (raw)
In-Reply-To: <20251030140355.4059-1-fmancera@suse.de>
On Thu, Oct 30, 2025 at 03:03:55PM +0100, Fernando Fernandez Mancera wrote:
> Since commit 30f241fcf52a ("xsk: Fix immature cq descriptor
> production"), the descriptor number is stored in skb control block and
> xsk_cq_submit_addr_locked() relies on it to put the umem addrs onto
> pool's completion queue.
>
> skb control block shouldn't be used for this purpose as after transmit
> xsk doesn't have control over it and other subsystems could use it. This
> leads to the following kernel panic due to a NULL pointer dereference.
>
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: Oops: 0000 [#1] SMP NOPTI
> CPU: 2 UID: 1 PID: 927 Comm: p4xsk.bin Not tainted 6.16.12+deb14-cloud-amd64 #1 PREEMPT(lazy) Debian 6.16.12-1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
> RIP: 0010:xsk_destruct_skb+0xd0/0x180
> [...]
> Call Trace:
> <IRQ>
> ? napi_complete_done+0x7a/0x1a0
> ip_rcv_core+0x1bb/0x340
> ip_rcv+0x30/0x1f0
> __netif_receive_skb_one_core+0x85/0xa0
> process_backlog+0x87/0x130
> __napi_poll+0x28/0x180
> net_rx_action+0x339/0x420
> handle_softirqs+0xdc/0x320
> ? handle_edge_irq+0x90/0x1e0
> do_softirq.part.0+0x3b/0x60
> </IRQ>
> <TASK>
> __local_bh_enable_ip+0x60/0x70
> __dev_direct_xmit+0x14e/0x1f0
> __xsk_generic_xmit+0x482/0xb70
> ? __remove_hrtimer+0x41/0xa0
> ? __xsk_generic_xmit+0x51/0xb70
> ? _raw_spin_unlock_irqrestore+0xe/0x40
> xsk_sendmsg+0xda/0x1c0
> __sys_sendto+0x1ee/0x200
> __x64_sys_sendto+0x24/0x30
> do_syscall_64+0x84/0x2f0
> ? __pfx_pollwake+0x10/0x10
> ? __rseq_handle_notify_resume+0xad/0x4c0
> ? restore_fpregs_from_fpstate+0x3c/0x90
> ? switch_fpu_return+0x5b/0xe0
> ? do_syscall_64+0x204/0x2f0
> ? do_syscall_64+0x204/0x2f0
> ? do_syscall_64+0x204/0x2f0
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> </TASK>
> [...]
> Kernel panic - not syncing: Fatal exception in interrupt
> Kernel Offset: 0x1c000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
>
> Instead use the skb destructor_arg pointer along with pointer tagging.
> As pointers are always aligned to 8B, use the bottom bit to indicate
> whether this a single address or an allocated struct containing several
> addresses.
>
> Fixes: 30f241fcf52a ("xsk: Fix immature cq descriptor production")
> Closes: https://lore.kernel.org/netdev/0435b904-f44f-48f8-afb0-68868474bf1c@nop.hu/
> Suggested-by: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Tested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Fernando thanks for stepping in and providing this fix!
And thanks Jakub for ptr tagging trick.
@BPF maintainers, please apply this patch.
> ---
> v2: remove some leftovers on skb_build and simplify fragmented traffic
> logic
>
> v3: drop skb extension approach, instead use pointer tagging in
> destructor_arg to know whether we have a single address or an allocated
> struct with multiple ones. Also, move from bpf to net as requested
>
> Note: tested with the crash reproducer and xdpsock tool
> ---
> net/xdp/xsk.c | 130 ++++++++++++++++++++++++++++----------------------
> 1 file changed, 74 insertions(+), 56 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 7b0c68a70888..d7354a3e2545 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -36,20 +36,13 @@
> #define TX_BATCH_SIZE 32
> #define MAX_PER_SOCKET_BUDGET 32
>
> -struct xsk_addr_node {
> - u64 addr;
> - struct list_head addr_node;
> -};
> -
> -struct xsk_addr_head {
> +struct xsk_addrs {
> u32 num_descs;
> - struct list_head addrs_list;
> + u64 addrs[MAX_SKB_FRAGS + 1];
> };
>
> static struct kmem_cache *xsk_tx_generic_cache;
>
> -#define XSKCB(skb) ((struct xsk_addr_head *)((skb)->cb))
> -
> void xsk_set_rx_need_wakeup(struct xsk_buff_pool *pool)
> {
> if (pool->cached_need_wakeup & XDP_WAKEUP_RX)
> @@ -558,29 +551,53 @@ static int xsk_cq_reserve_locked(struct xsk_buff_pool *pool)
> return ret;
> }
>
> +static bool xsk_skb_destructor_is_addr(struct sk_buff *skb)
> +{
> + return (uintptr_t)skb_shinfo(skb)->destructor_arg & 0x1UL;
> +}
> +
> +static u64 xsk_skb_destructor_get_addr(struct sk_buff *skb)
> +{
> + return (u64)((uintptr_t)skb_shinfo(skb)->destructor_arg & ~0x1UL);
> +}
> +
> +static u32 xsk_get_num_desc(struct sk_buff *skb)
> +{
> + struct xsk_addrs *xsk_addr;
> +
> + if (xsk_skb_destructor_is_addr(skb))
> + return 1;
> +
> + xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> +
> + return xsk_addr->num_descs;
> +}
> +
> static void xsk_cq_submit_addr_locked(struct xsk_buff_pool *pool,
> struct sk_buff *skb)
> {
> - struct xsk_addr_node *pos, *tmp;
> + u32 num_descs = xsk_get_num_desc(skb);
> + struct xsk_addrs *xsk_addr;
> u32 descs_processed = 0;
> unsigned long flags;
> - u32 idx;
> + u32 idx, i;
>
> spin_lock_irqsave(&pool->cq_lock, flags);
> idx = xskq_get_prod(pool->cq);
>
> - xskq_prod_write_addr(pool->cq, idx,
> - (u64)(uintptr_t)skb_shinfo(skb)->destructor_arg);
> - descs_processed++;
> + if (unlikely(num_descs > 1)) {
> + xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
>
> - if (unlikely(XSKCB(skb)->num_descs > 1)) {
> - list_for_each_entry_safe(pos, tmp, &XSKCB(skb)->addrs_list, addr_node) {
> + for (i = 0; i < num_descs; i++) {
> xskq_prod_write_addr(pool->cq, idx + descs_processed,
> - pos->addr);
> + xsk_addr->addrs[i]);
> descs_processed++;
> - list_del(&pos->addr_node);
> - kmem_cache_free(xsk_tx_generic_cache, pos);
> }
> + kmem_cache_free(xsk_tx_generic_cache, xsk_addr);
> + } else {
> + xskq_prod_write_addr(pool->cq, idx,
> + xsk_skb_destructor_get_addr(skb));
> + descs_processed++;
> }
> xskq_prod_submit_n(pool->cq, descs_processed);
> spin_unlock_irqrestore(&pool->cq_lock, flags);
> @@ -595,16 +612,6 @@ static void xsk_cq_cancel_locked(struct xsk_buff_pool *pool, u32 n)
> spin_unlock_irqrestore(&pool->cq_lock, flags);
> }
>
> -static void xsk_inc_num_desc(struct sk_buff *skb)
> -{
> - XSKCB(skb)->num_descs++;
> -}
> -
> -static u32 xsk_get_num_desc(struct sk_buff *skb)
> -{
> - return XSKCB(skb)->num_descs;
> -}
> -
> static void xsk_destruct_skb(struct sk_buff *skb)
> {
> struct xsk_tx_metadata_compl *compl = &skb_shinfo(skb)->xsk_meta;
> @@ -621,27 +628,22 @@ static void xsk_destruct_skb(struct sk_buff *skb)
> static void xsk_skb_init_misc(struct sk_buff *skb, struct xdp_sock *xs,
> u64 addr)
> {
> - BUILD_BUG_ON(sizeof(struct xsk_addr_head) > sizeof(skb->cb));
> - INIT_LIST_HEAD(&XSKCB(skb)->addrs_list);
> skb->dev = xs->dev;
> skb->priority = READ_ONCE(xs->sk.sk_priority);
> skb->mark = READ_ONCE(xs->sk.sk_mark);
> - XSKCB(skb)->num_descs = 0;
> skb->destructor = xsk_destruct_skb;
> - skb_shinfo(skb)->destructor_arg = (void *)(uintptr_t)addr;
> + skb_shinfo(skb)->destructor_arg = (void *)((uintptr_t)addr | 0x1UL);
> }
>
> static void xsk_consume_skb(struct sk_buff *skb)
> {
> struct xdp_sock *xs = xdp_sk(skb->sk);
> u32 num_descs = xsk_get_num_desc(skb);
> - struct xsk_addr_node *pos, *tmp;
> + struct xsk_addrs *xsk_addr;
>
> if (unlikely(num_descs > 1)) {
> - list_for_each_entry_safe(pos, tmp, &XSKCB(skb)->addrs_list, addr_node) {
> - list_del(&pos->addr_node);
> - kmem_cache_free(xsk_tx_generic_cache, pos);
> - }
> + xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> + kmem_cache_free(xsk_tx_generic_cache, xsk_addr);
> }
>
> skb->destructor = sock_wfree;
> @@ -701,7 +703,6 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> {
> struct xsk_buff_pool *pool = xs->pool;
> u32 hr, len, ts, offset, copy, copied;
> - struct xsk_addr_node *xsk_addr;
> struct sk_buff *skb = xs->skb;
> struct page *page;
> void *buffer;
> @@ -727,16 +728,27 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> return ERR_PTR(err);
> }
> } else {
> - xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache, GFP_KERNEL);
> - if (!xsk_addr)
> - return ERR_PTR(-ENOMEM);
> + struct xsk_addrs *xsk_addr;
> +
> + if (xsk_skb_destructor_is_addr(skb)) {
> + xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache,
> + GFP_KERNEL);
> + if (!xsk_addr)
> + return ERR_PTR(-ENOMEM);
> +
> + xsk_addr->num_descs = 1;
> + xsk_addr->addrs[0] = xsk_skb_destructor_get_addr(skb);
> + skb_shinfo(skb)->destructor_arg = (void *)xsk_addr;
> + } else {
> + xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> + }
>
> /* in case of -EOVERFLOW that could happen below,
> * xsk_consume_skb() will release this node as whole skb
> * would be dropped, which implies freeing all list elements
> */
> - xsk_addr->addr = desc->addr;
> - list_add_tail(&xsk_addr->addr_node, &XSKCB(skb)->addrs_list);
> + xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
> + xsk_addr->num_descs++;
> }
>
> len = desc->len;
> @@ -813,7 +825,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> }
> } else {
> int nr_frags = skb_shinfo(skb)->nr_frags;
> - struct xsk_addr_node *xsk_addr;
> + struct xsk_addrs *xsk_addr;
> struct page *page;
> u8 *vaddr;
>
> @@ -828,11 +840,20 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> goto free_err;
> }
>
> - xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache, GFP_KERNEL);
> - if (!xsk_addr) {
> - __free_page(page);
> - err = -ENOMEM;
> - goto free_err;
> + if (xsk_skb_destructor_is_addr(skb)) {
> + xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache,
> + GFP_KERNEL);
> + if (!xsk_addr) {
> + __free_page(page);
> + err = -ENOMEM;
> + goto free_err;
> + }
> +
> + xsk_addr->num_descs = 1;
> + xsk_addr->addrs[0] = xsk_skb_destructor_get_addr(skb);
> + skb_shinfo(skb)->destructor_arg = (void *)xsk_addr;
> + } else {
> + xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> }
>
> vaddr = kmap_local_page(page);
> @@ -842,13 +863,11 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> skb_add_rx_frag(skb, nr_frags, page, 0, len, PAGE_SIZE);
> refcount_add(PAGE_SIZE, &xs->sk.sk_wmem_alloc);
>
> - xsk_addr->addr = desc->addr;
> - list_add_tail(&xsk_addr->addr_node, &XSKCB(skb)->addrs_list);
> + xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
> + xsk_addr->num_descs++;
> }
> }
>
> - xsk_inc_num_desc(skb);
> -
> return skb;
>
> free_err:
> @@ -857,7 +876,6 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>
> if (err == -EOVERFLOW) {
> /* Drop the packet */
> - xsk_inc_num_desc(xs->skb);
> xsk_drop_skb(xs->skb);
> xskq_cons_release(xs->tx);
> } else {
> @@ -1904,7 +1922,7 @@ static int __init xsk_init(void)
> goto out_pernet;
>
> xsk_tx_generic_cache = kmem_cache_create("xsk_generic_xmit_cache",
> - sizeof(struct xsk_addr_node),
> + sizeof(struct xsk_addrs),
> 0, SLAB_HWCACHE_ALIGN, NULL);
> if (!xsk_tx_generic_cache) {
> err = -ENOMEM;
> --
> 2.51.0
>
next prev parent reply other threads:[~2025-11-17 16:08 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-30 14:03 [PATCH net v3] xsk: avoid data corruption on cq descriptor number Fernando Fernandez Mancera
2025-10-31 9:51 ` Jason Xing
2025-10-31 10:05 ` Fernando Fernandez Mancera
2025-10-31 10:23 ` Jason Xing
2025-10-31 10:37 ` Fernando Fernandez Mancera
2025-11-17 16:08 ` Maciej Fijalkowski [this message]
2025-11-17 16:11 ` Fernando Fernandez Mancera
2025-11-17 23:59 ` Jakub Kicinski
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