Re: [PATCH net] net/mlx5: Fix double unregister of HCA_PORTS component

[Farhan Ali <alifm@linux.ibm.com>]

On 12/2/2025 3:12 AM, Gerd Bayer wrote:
> Clear hca_devcom_comp in device's private data after unregistering it in
> LAG teardown. Otherwise a slightly lagging second pass through
> mlx5_unload_one() might try to unregister it again and trip over
> use-after-free.
>
> On s390 almost all PCI level recovery events trigger two passes through
> mxl5_unload_one() - one through the poll_health() method and one through
> mlx5_pci_err_detected() as callback from generic PCI error recovery.
> While testing PCI error recovery paths with more kernel debug features
> enabled, this issue reproducibly led to kernel panics with the following
> call chain:
>
>   Unable to handle kernel pointer dereference in virtual kernel address space
>   Failing address: 6b6b6b6b6b6b6000 TEID: 6b6b6b6b6b6b6803 ESOP-2 FSI
>   Fault in home space mode while using kernel ASCE.
>   AS:00000000705c4007 R3:0000000000000024
>   Oops: 0038 ilc:3 [#1]SMP
>
>   CPU: 14 UID: 0 PID: 156 Comm: kmcheck Kdump: loaded Not tainted
>        6.18.0-20251130.rc7.git0.16131a59cab1.300.fc43.s390x+debug #1 PREEMPT
>
>   Krnl PSW : 0404e00180000000 0000020fc86aa1dc (__lock_acquire+0x5c/0x15f0)
>              R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
>   Krnl GPRS: 0000000000000000 0000020f00000001 6b6b6b6b6b6b6c33 0000000000000000
>              0000000000000000 0000000000000000 0000000000000001 0000000000000000
>              0000000000000000 0000020fca28b820 0000000000000000 0000010a1ced8100
>              0000010a1ced8100 0000020fc9775068 0000018fce14f8b8 0000018fce14f7f8
>   Krnl Code: 0000020fc86aa1cc: e3b003400004        lg      %r11,832
>              0000020fc86aa1d2: a7840211           brc     8,0000020fc86aa5f4
>             *0000020fc86aa1d6: c09000df0b25       larl    %r9,0000020fca28b820
>             >0000020fc86aa1dc: d50790002000       clc     0(8,%r9),0(%r2)
>              0000020fc86aa1e2: a7840209           brc     8,0000020fc86aa5f4
>              0000020fc86aa1e6: c0e001100401       larl    %r14,0000020fca8aa9e8
>              0000020fc86aa1ec: c01000e25a00       larl    %r1,0000020fca2f55ec
>              0000020fc86aa1f2: a7eb00e8           aghi    %r14,232
>
>   Call Trace:
>    __lock_acquire+0x5c/0x15f0
>    lock_acquire.part.0+0xf8/0x270
>    lock_acquire+0xb0/0x1b0
>    down_write+0x5a/0x250
>    mlx5_detach_device+0x42/0x110 [mlx5_core]
>    mlx5_unload_one_devl_locked+0x50/0xc0 [mlx5_core]
>    mlx5_unload_one+0x42/0x60 [mlx5_core]
>    mlx5_pci_err_detected+0x94/0x150 [mlx5_core]
>    zpci_event_attempt_error_recovery+0xcc/0x388
>
> Fixes: 5a977b5833b7 ("net/mlx5: Lag, move devcom registration to LAG layer")
> Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
> ---
> Hi Shay et al,
>
> while checking for potential regressions by Lukas Wunner's recent work
> on pci_save/restore_state() for the recoverability of mlx5 functions I
> consistently hit this bug. (Bjorn has queued this up for 6.19, according
> to [0] and [1])
>
> Apparently, the issue is unrelated to Lukas' work but can be reproduced
> with master. It appears to be timing-sensitive, since it shows up only
> when I use s390's debug_defconfig, but I think needs fixing anyhow, as
> timing can change for other reasons, too.
>
> I've spotted two additional places where the devcom reference is not
> cleared after calling mlx5_devcom_unregister_component() in
> drivers/net/ethernet/mellanox/mlx5/core/lib/sd.c that I have not
> addressed with a patch, since I'm unclear about how to test these
> paths.
>
> Thanks,
> Gerd
>
> [0] https://lore.kernel.org/all/cover.1760274044.git.lukas@wunner.de/
> [1] https://lore.kernel.org/linux-pci/cover.1763483367.git.lukas@wunner.de/
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/lag/lag.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lag/lag.c b/drivers/net/ethernet/mellanox/mlx5/core/lag/lag.c
> index 3db0387bf6dcb727a65df9d0253f242554af06db..8ec04a5f434dd4f717d6d556649fcc2a584db847 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/lag/lag.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/lag/lag.c
> @@ -1413,6 +1413,7 @@ static int __mlx5_lag_dev_add_mdev(struct mlx5_core_dev *dev)
>   static void mlx5_lag_unregister_hca_devcom_comp(struct mlx5_core_dev *dev)
>   {
>   	mlx5_devcom_unregister_component(dev->priv.hca_devcom_comp);
> +	dev->priv.hca_devcom_comp = NULL;
>   }

Though this fix looks correct to me in freeing hca_devcom_comp (not too 
familiar with mlx5 internals), I wonder if it would be better to just 
set devcom = NULL in devcom_free_comp_dev() after the kfree? This would 
also take care of other places where devcom is not set to NULL?

Thanks

Farhan