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Message-ID: <01a09fe7-9f58-4fc5-a84d-12d5b4b92bbd@suse.de>
Date: Thu, 20 Nov 2025 14:16:05 +0100
From: Fernando Fernandez Mancera <fmancera@...e.de>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: netdev@...r.kernel.org, csmate@....hu, maciej.fijalkowski@...el.com,
bpf@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, sdf@...ichev.me,
hawk@...nel.org, daniel@...earbox.net, ast@...nel.org,
john.fastabend@...il.com, magnus.karlsson@...el.com
Subject: Re: [PATCH net v5] xsk: avoid data corruption on cq descriptor number
On 11/20/25 1:56 PM, Jason Xing wrote:
> On Thu, Nov 20, 2025 at 7:02 PM Fernando Fernandez Mancera
> <fmancera@...e.de> 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@...nel.org>
>> Signed-off-by: Fernando Fernandez Mancera <fmancera@...e.de>
>> ---
>> 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
>>
>> v4: repost after rebasing
>>
>> v5: fixed increase logic so -EOVERFLOW is handled correctly as
>> suggested by Jason. Also dropped the acks/reviewed tags as code changed.
>> ---
>> net/xdp/xsk.c | 141 ++++++++++++++++++++++++++++++--------------------
>> 1 file changed, 85 insertions(+), 56 deletions(-)
>>
>> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
>> index 7b0c68a70888..f87cc4c89339 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 +551,63 @@ 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 void xsk_inc_num_desc(struct sk_buff *skb)
>> +{
>> + struct xsk_addrs *xsk_addr;
>> +
>> + if (!xsk_skb_destructor_is_addr(skb)) {
>
> It's the condition that causes the above issues. Please see the
> following comment.
>
>> + xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
>> + xsk_addr->num_descs++;
>> + }
>> +}
>> +
>> +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 +622,6 @@ 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 +638,22 @@ 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 +713,6 @@ 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 +738,26 @@ 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;
>
> At this point, actually @num_descs should be equal to 2. I know it
> will be incremented by one at the end of xsk_build_skb().My concern
Why? if we reach this it means this is the first time we see fragmented
traffic therefore we allocate xsk_addrs struct, store the previous umem
address in addrs[0] and num_descs = 1 and finally if no -EOVERFLOW
happens then the new desc->addr is added to addrs[num_descs] (which is
addrs[1]).
Later, at the end of xsk_build_skb() or if -EOVERFLOW happens we
increase num_descs so if xsk_cq_cancel_locked() or
xsk_cq_submit_addr_locked() is called we have the right number of
descriptors.
If we set @num_descs to 2 here, then when do we increase? I do not
understand that.
> is when skb only carries one descriptor, I don't see any place setting
> @num_descs to 1?
>
When skb carries only one descriptor i.e traffic isn't segmented then
xsk_addr struct isn't allocated and destructor_arg is carrying just an
umem address.
This is why xsk_get_num_desc() returns 1 if destructor_arg is an umem
address, because it means there is just a single descriptor.
>> + 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;
>> }
>>
>> len = desc->len;
>> @@ -813,10 +834,25 @@ 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;
>>
>> + if (xsk_skb_destructor_is_addr(skb)) {
>> + xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache,
>> + GFP_KERNEL);
>> + if (!xsk_addr) {
>> + err = -ENOMEM;
>> + goto free_err;
>> + }
>> +
>> + xsk_addr->num_descs = 1;
>
> same for here.
>
>> + 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;
>> + }
>> +
>> if (unlikely(nr_frags == (MAX_SKB_FRAGS - 1) && xp_mb_desc(desc))) {
>> err = -EOVERFLOW;
>> goto free_err;
>> @@ -828,13 +864,6 @@ 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;
>> - }
>> -
>> vaddr = kmap_local_page(page);
>> memcpy(vaddr, buffer, len);
>> kunmap_local(vaddr);
>> @@ -842,12 +871,12 @@ 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_inc_num_desc(skb);
>> + if (!xsk_skb_destructor_is_addr(skb))
>
> nit: duplicate if statement
>
> IIUC, I'm afraid you have to repost this patch after 24 hour...
>
Thanks, yes this if statement isn't necessary. Thanks! I will repost
after 24 hours.
> Thanks,
> Jason
>
>> + xsk_inc_num_desc(skb);
>>
>> return skb;
>>
>> @@ -1904,7 +1933,7 @@ 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;
>> --
>> 2.51.1
>>
>
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