Subject: Re: [PATCH net v3] ixgbe: Panic during XDP_TX with > 64 CPUs

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 5 Apr 2023 16:40:33 +0200

On Wed, Mar 08, 2023 at 02:07:57PM -0800, John Hickey wrote:
> In commit 'ixgbe: let the xdpdrv work with more than 64 cpus'
> (4fe815850bdc), support was added to allow XDP programs to run on systems
> with more than 64 CPUs by locking the XDP TX rings and indexing them
> using cpu % 64 (IXGBE_MAX_XDP_QS).
> 
> Upon trying this out patch via the Intel 5.18.6 out of tree driver
> on a system with more than 64 cores, the kernel paniced with an
> array-index-out-of-bounds at the return in ixgbe_determine_xdp_ring in
> ixgbe.h, which means ixgbe_determine_xdp_q_idx was just returning the
> cpu instead of cpu % IXGBE_MAX_XDP_QS.  An example splat:

i am assuming that in-tree driver is also affected so the info about OOT
driver is irrelevant here :)

> 
>  ==========================================================================
>  UBSAN: array-index-out-of-bounds in
>  /var/lib/dkms/ixgbe/5.18.6+focal-1/build/src/ixgbe.h:1147:26
>  index 65 is out of range for type 'ixgbe_ring *[64]'
>  ==========================================================================
>  BUG: kernel NULL pointer dereference, address: 0000000000000058
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 0 P4D 0
>  Oops: 0000 [#1] SMP NOPTI
>  CPU: 65 PID: 408 Comm: ksoftirqd/65
>  Tainted: G          IOE     5.15.0-48-generic #54~20.04.1-Ubuntu
>  Hardware name: Dell Inc. PowerEdge R640/0W23H8, BIOS 2.5.4 01/13/2020
>  RIP: 0010:ixgbe_xmit_xdp_ring+0x1b/0x1c0 [ixgbe]
>  Code: 3b 52 d4 cf e9 42 f2 ff ff 66 0f 1f 44 00 00 0f 1f 44 00 00 55 b9
>  00 00 00 00 48 89 e5 41 57 41 56 41 55 41 54 53 48 83 ec 08 <44> 0f b7
>  47 58 0f b7 47 5a 0f b7 57 54 44 0f b7 76 08 66 41 39 c0
>  RSP: 0018:ffffbc3fcd88fcb0 EFLAGS: 00010282
>  RAX: ffff92a253260980 RBX: ffffbc3fe68b00a0 RCX: 0000000000000000
>  RDX: ffff928b5f659000 RSI: ffff928b5f659000 RDI: 0000000000000000
>  RBP: ffffbc3fcd88fce0 R08: ffff92b9dfc20580 R09: 0000000000000001
>  R10: 3d3d3d3d3d3d3d3d R11: 3d3d3d3d3d3d3d3d R12: 0000000000000000
>  R13: ffff928b2f0fa8c0 R14: ffff928b9be20050 R15: 000000000000003c
>  FS:  0000000000000000(0000) GS:ffff92b9dfc00000(0000)
>  knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000000058 CR3: 000000011dd6a002 CR4: 00000000007706e0
>  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>  PKRU: 55555554
>  Call Trace:
>   <TASK>
>   ixgbe_poll+0x103e/0x1280 [ixgbe]
>   ? sched_clock_cpu+0x12/0xe0
>   __napi_poll+0x30/0x160
>   net_rx_action+0x11c/0x270
>   __do_softirq+0xda/0x2ee
>   run_ksoftirqd+0x2f/0x50
>   smpboot_thread_fn+0xb7/0x150
>   ? sort_range+0x30/0x30
>   kthread+0x127/0x150
>   ? set_kthread_struct+0x50/0x50
>   ret_from_fork+0x1f/0x30
>   </TASK>
> 
> I think this is how it happens:
> 
> Upon loading the first XDP program on a system with more than 64 CPUs,
> ixgbe_xdp_locking_key is incremented in ixgbe_xdp_setup.  However,
> immediately after this, the rings are reconfigured by ixgbe_setup_tc.
> ixgbe_setup_tc calls ixgbe_clear_interrupt_scheme which calls
> ixgbe_free_q_vectors which calls ixgbe_free_q_vector in a loop.
> ixgbe_free_q_vector decrements ixgbe_xdp_locking_key once per call if
> it is non-zero.  Commenting out the decrement in ixgbe_free_q_vector
> stopped my system from panicing.
> 
> I suspect to make the original patch work, I would need to load an XDP
> program and then replace it in order to get ixgbe_xdp_locking_key back
> above 0 since ixgbe_setup_tc is only called when transitioning between
> XDP and non-XDP ring configurations, while ixgbe_xdp_locking_key is
> incremented every time ixgbe_xdp_setup is called.
> 
> Also, ixgbe_setup_tc can be called via ethtool --set-channels, so this
> becomes another path to decrement ixgbe_xdp_locking_key to 0 on systems
> with greater than 64 CPUs.
> 
> For this patch, I have changed static_branch_inc to static_branch_enable
> in ixgbe_setup_xdp.  We weren't counting references.  The
> ixgbe_xdp_locking_key only protects code in the XDP_TX path, which is
> not run when an XDP program is loaded.  The other condition for setting
> it on is the number of CPUs, which I assume is static.

>From technical POV i think the contents here are fine. Maybe we could even
move this check to probe path because as you said this is static. I saw
that others had some wording/style comments, so if you decide to fix it
and push a v4, then you can add:

Reviewed-by: Maciej Fijalkowski <[email-masked]>

> 
> Fixes: 4fe815850bdc ("ixgbe: let the xdpdrv work with more than 64 cpus")
> Signed-off-by: John Hickey <[email-masked]>
> ---
> v1 -> v2:
> 	Added Fixes and net tag.  No code changes.
> v2 -> v3:
> 	Added splat.  Slight clarification as to why ixgbe_xdp_locking_key
> 	is not turned off.  Based on feedback from Maciej Fijalkowski.
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c  | 3 ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
>  2 files changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> index f8156fe4b1dc..0ee943db3dc9 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> @@ -1035,9 +1035,6 @@ static void ixgbe_free_q_vector(struct ixgbe_adapter *adapter, int v_idx)
>  	adapter->q_vector[v_idx] = NULL;
>  	__netif_napi_del(&q_vector->napi);
>  
> -	if (static_key_enabled(&ixgbe_xdp_locking_key))
> -		static_branch_dec(&ixgbe_xdp_locking_key);
> -
>  	/*
>  	 * after a call to __netif_napi_del() napi may still be used and
>  	 * ixgbe_get_stats64() might access the rings on this vector,
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index ab8370c413f3..cd2fb72c67be 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -10283,7 +10283,7 @@ static int ixgbe_xdp_setup(struct net_device *dev, struct bpf_prog *prog)
>  	if (nr_cpu_ids > IXGBE_MAX_XDP_QS * 2)
>  		return -ENOMEM;
>  	else if (nr_cpu_ids > IXGBE_MAX_XDP_QS)
> -		static_branch_inc(&ixgbe_xdp_locking_key);
> +		static_branch_enable(&ixgbe_xdp_locking_key);
>  
>  	old_prog = xchg(&adapter->xdp_prog, prog);
>  	need_reset = (!!prog != !!old_prog);
> -- 
> 2.37.2
>