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Message-ID: <CAL+tcoBueigrGnKASad7XFybXMHvj5jAOcZS8_bY3J-7XVZShQ@mail.gmail.com>
Date: Thu, 20 Nov 2025 21:40:27 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Fernando Fernandez Mancera <fmancera@...e.de>
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 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.
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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