Re: [PATCH v1 1/2] arm_mpam: Fix null pointer dereference when restoring bandwidth counters

[James Morse <james.morse@arm.com>]

Hi Ben,

On 27/02/2026 11:03, Ben Horgan wrote:
> When an MSC supporting memory bandwidth monitoring is brought offline and
> then online, mpam_restore_mbwu_state() calls __ris_msmon_read() via ipi to
> restore the configuration of the bandwidth counters. It doesn't care about
> the value read, mbwu_arg.val, and doesn't set it leading to a null pointer

If we're lucky! If its not NULL, it'll take it as a pointer and go corrupt
something else.


> dereference when __ris_msmon_read() adds to it. This results in a kernel
> oops with a call trace such as:
> 
> Call trace:
> __ris_msmon_read+0x19c/0x64c (P)
> mpam_restore_mbwu_state+0xa0/0xe8
> smp_call_on_cpu_callback+0x1c/0x38
> process_one_work+0x154/0x4b4
> worker_thread+0x188/0x310
> kthread+0x11c/0x130
> ret_from_fork+0x10/0x20
> 
> Provide a local variable for val to avoid __ris_msmon_read() dereferencing
> a null pointer when adding to val.

> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 1eebc2602187..0666be6b0e88 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -1428,6 +1428,7 @@ static void mpam_reprogram_ris_partid(struct mpam_msc_ris *ris, u16 partid,
>  static int mpam_restore_mbwu_state(void *_ris)
>  {
>  	int i;
> +	u64 val;
>  	struct mon_read mwbu_arg;
>  	struct mpam_msc_ris *ris = _ris;
>  	struct mpam_class *class = ris->vmsc->comp->class;
> @@ -1437,6 +1438,7 @@ static int mpam_restore_mbwu_state(void *_ris)
>  			mwbu_arg.ris = ris;
>  			mwbu_arg.ctx = &ris->mbwu_state[i].cfg;
>  			mwbu_arg.type = mpam_msmon_choose_counter(class);
> +			mwbu_arg.val = &val;
>  
>  			__ris_msmon_read(&mwbu_arg);
>  		}


Reviewed-by: James Morse <james.morse@arm.com>


Thanks for catching this!

James