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Message-ID: <c68423d1-d4e6-4d13-973b-44a791a3c806@suse.de>
Date: Thu, 20 Nov 2025 14:47:07 +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 2:40 PM, Jason Xing wrote:
> On Thu, Nov 20, 2025 at 9:16 PM Fernando Fernandez Mancera
> <fmancera@...e.de> wrote:
>>
>> 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.
>
> I'm not saying the above logic is not right :)
>
>>
>>> 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.
>
> Here, It's the case that I'm worried about.
>
> Ah, well, I see your point. I previously thought this function would
> return @num_descs directly.
>
> Surely it works. However, from my perspective I feel it's a bit weird
> because when the skb only carries one desc, the @num_descs remains
> zero which doesn't reflect the fact. I understand you use that
> function to return one instead of reading @num_descs in this case.
> Just a bit weird. I'm not sure what Maciej's opinion is here.
>
FWIW, @num_descs isn't zero it just doesn't exist. num_descs is a field
of xsk_addr struct which is only allocated for fragmented traffic. This
is why xsk_get_num_desc() must be always used.
Thanks,
Fernando.
> Anyway, thanks as always for fixing this:
> Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
>
> Thanks,
> Jason
>
>
>>
>>>> + 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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