Subject: Re: [PATCH net v2 1/1] i40e: xsk: advance next_to_clean on status descriptors

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 22 Oct 2025 19:17:20 +0200

On Wed, Oct 22, 2025 at 12:32:00AM +0700, Alessandro Decina wrote:

Hi Alessandro,

> Whenever a status descriptor is received, i40e processes and skips over
> it, correctly updating next_to_process but forgetting to update
> next_to_clean. In the next iteration this accidentally causes the
> creation of an invalid multi-buffer xdp_buff where the first fragment
> is the status descriptor.
> 
> If then a skb is constructed from such an invalid buffer - because the
> eBPF program returns XDP_PASS - a panic occurs:

can you elaborate on the test case that would reproduce this? I suppose
AF_XDP ZC with jumbo frames, doing XDP_PASS, but what was FDIR setup that
caused status descriptors?

> 
> [ 5866.367317] BUG: unable to handle page fault for address: ffd31c37eab1c980
> [ 5866.375050] #PF: supervisor read access in kernel mode
> [ 5866.380825] #PF: error_code(0x0000) - not-present page
> [ 5866.386602] PGD 0
> [ 5866.388867] Oops: Oops: 0000 [#1] SMP NOPTI
> [ 5866.393575] CPU: 34 UID: 0 PID: 0 Comm: swapper/34 Not tainted 6.17.0-custom #1 PREEMPT(voluntary)
> [ 5866.403740] Hardware name: Supermicro AS -2115GT-HNTR/H13SST-G, BIOS 3.2 03/20/2025
> [ 5866.412339] RIP: 0010:memcpy+0x8/0x10
> [ 5866.416454] Code: cc cc 90 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 90 48 89 f8 48 89 d1 <f3> a4 e9 fc 26 c0 fe 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90
> [ 5866.437538] RSP: 0018:ff428d9ec0bb0ca8 EFLAGS: 00010286
> [ 5866.443415] RAX: ff2dd26dbd8f0000 RBX: ff2dd265ad161400 RCX: 00000000000004e1
> [ 5866.451435] RDX: 00000000000004e1 RSI: ffd31c37eab1c980 RDI: ff2dd26dbd8f0000
> [ 5866.459454] RBP: ff428d9ec0bb0d40 R08: 0000000000000000 R09: 0000000000000000
> [ 5866.467470] R10: 0000000000000000 R11: 0000000000000000 R12: ff428d9eec726ef8
> [ 5866.475490] R13: ff2dd26dbd8f0000 R14: ff2dd265ca2f9fc0 R15: ff2dd26548548b80
> [ 5866.483509] FS:  0000000000000000(0000) GS:ff2dd2c363592000(0000) knlGS:0000000000000000
> [ 5866.492600] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 5866.499060] CR2: ffd31c37eab1c980 CR3: 0000000178d7b040 CR4: 0000000000f71ef0
> [ 5866.507079] PKRU: 55555554
> [ 5866.510125] Call Trace:
> [ 5866.512867]  <IRQ>
> [ 5866.515132]  ? i40e_clean_rx_irq_zc+0xc50/0xe60 [i40e]
> [ 5866.520921]  i40e_napi_poll+0x2d8/0x1890 [i40e]
> [ 5866.526022]  ? srso_alias_return_thunk+0x5/0xfbef5
> [ 5866.531408]  ? raise_softirq+0x24/0x70
> [ 5866.535623]  ? srso_alias_return_thunk+0x5/0xfbef5
> [ 5866.541011]  ? srso_alias_return_thunk+0x5/0xfbef5
> [ 5866.546397]  ? rcu_sched_clock_irq+0x225/0x1800
> [ 5866.551493]  __napi_poll+0x30/0x230
> [ 5866.555423]  net_rx_action+0x20b/0x3f0
> [ 5866.559643]  handle_softirqs+0xe4/0x340
> [ 5866.563962]  __irq_exit_rcu+0x10e/0x130
> [ 5866.568283]  irq_exit_rcu+0xe/0x20
> [ 5866.572110]  common_interrupt+0xb6/0xe0
> [ 5866.576425]  </IRQ>
> [ 5866.578791]  <TASK>
> 
> Advance next_to_clean to ensure invalid xdp_buff(s) aren't created.
> 
> Fixes: 1c9ba9c14658 ("i40e: xsk: add RX multi-buffer support")
> Signed-off-by: Alessandro Decina <[email-masked]>
> ---
>  drivers/net/ethernet/intel/i40e/i40e_xsk.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> index 9f47388eaba5..dbc19083bbb7 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> @@ -441,13 +441,18 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int budget)
>  		dma_rmb();
>  
>  		if (i40e_rx_is_programming_status(qword)) {
> +			u16 ntp;
> +
>  			i40e_clean_programming_status(rx_ring,
>  						      rx_desc->raw.qword[0],
>  						      qword);
>  			bi = *i40e_rx_bi(rx_ring, next_to_process);
>  			xsk_buff_free(bi);
> -			if (++next_to_process == count)
> +			ntp = next_to_process++;
> +			if (next_to_process == count)
>  				next_to_process = 0;
> +			if (next_to_clean == ntp)
> +				next_to_clean = next_to_process;

I wonder if this is more readable?

diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
index 9f47388eaba5..36f412a2d836 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
@@ -446,6 +446,10 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int budget)
 						      qword);
 			bi = *i40e_rx_bi(rx_ring, next_to_process);
 			xsk_buff_free(bi);
+			if (next_to_clean == next_to_process) {
+				if (++next_to_clean == count)
+					next_to_clean = 0;
+			}
 			if (++next_to_process == count)
 				next_to_process = 0;
 			continue;

>  			continue;
>  		}
> -- 
> 2.43.0