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Message-ID: <CAL+tcoAE6_15tOjZFrdif1ixBja3_qeUKL2GUvOyypcimLFJXw@mail.gmail.com>
Date: Thu, 20 Nov 2025 21:55:32 +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 9:47 PM Fernando Fernandez Mancera
<fmancera@...e.de> wrote:
>
> 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.
Right, I'm totally fine with it. And I don't think you need to repost
it with that minor change unless Maciej asked you to do so :)
Thanks,
Jason
>
> 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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