Re: [PATCH] arm64: smp: do not allocate CPU IDs to invalid CPU nodes

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Jun 21, 2024 at 03:50:45PM +0800, Lingyue wrote:
> Many modules, such as arch topology, rely on num_possible_cpus() to
> allocate memory and then access the allocated space using CPU IDs.
> These modules assume that there are no gaps in cpu_possible_mask.

Is there any documented requirement that cpu_possible_mask has no gaps?

It looks like other architectures can have gaps in their
cpu_possible_mask, there's no documented requiremetns AFAICT, and there
are a bunch of commits handling cpu_possible_mask having gaps, e.g.

  bc75e99983df1efd ("rcu: Correctly handle sparse possible cpus")
  3da43104d3187184 ("ARC: Adjust cpuinfo for non-continuous cpu ids")
  72917235fd5f0863 ("tracing: Fix for non-continuous cpu ids")

... so I don't think that the topology code should assume that there are
no gaps in cpu_possible_mask.

> However, in of_parse_and_init_cpus(), CPU IDs are still allocated
> for invalid CPU nodes, leading to gaps in cpu_possible_mask and
> resulting in out-of-bounds memory access. So it is crucial to avoid
> allocating CPU IDs to invalid CPU nodes.

AFAICT the topology code could use 'nr_cpu_ids' instead of
'nr_possible_cpus()', like the tracing commit above, or it could use a
per-cpu allocation to avoid this.

> This issue can be reproduced easily on QEMU with KASAN enabled, by
> modifing reg property of a CPU node to 0xFFFFFFFF
> 
> [    0.197756] BUG: KASAN: slab-out-of-bounds in topology_normalize_cpu_scale.part.0+0x2cc/0x34c
> [    0.199518] Read of size 4 at addr ffff000007ebe924 by task swapper/0/1
> [    0.200087]
> [    0.200739] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.10.0-rc4 #3
> [    0.201647] Hardware name: linux,dummy-virt (DT)
> [    0.203067] Call trace:
> [    0.203404]  dump_backtrace+0x90/0xe8
> [    0.203974]  show_stack+0x18/0x24
> [    0.204424]  dump_stack_lvl+0x78/0x90
> [    0.205090]  print_report+0x114/0x5cc
> [    0.205908]  kasan_report+0xa4/0xf0
> [    0.206488]  __asan_report_load4_noabort+0x20/0x2c
> [    0.207427]  topology_normalize_cpu_scale.part.0+0x2cc/0x34c
> [    0.208275]  init_cpu_topology+0x254/0x430
> [    0.209518]  smp_prepare_cpus+0x20/0x25c
> [    0.210824]  kernel_init_freeable+0x1dc/0x4fc
> [    0.212047]  kernel_init+0x24/0x1ec
> [    0.213143]  ret_from_fork+0x10/0x20
> 
> Signed-off-by: Lingyue <lingyue@xiaomi.com>
> ---
>  arch/arm64/kernel/smp.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 31c8b3094dd7..5b4178145920 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -638,12 +638,12 @@ static void __init of_parse_and_init_cpus(void)
>  		u64 hwid = of_get_cpu_hwid(dn, 0);
>  
>  		if (hwid & ~MPIDR_HWID_BITMASK)
> -			goto next;
> +			continue;
>  
>  		if (is_mpidr_duplicate(cpu_count, hwid)) {
>  			pr_err("%pOF: duplicate cpu reg properties in the DT\n",
>  				dn);
> -			goto next;
> +			continue;
>  		}
>  
>  		/*
> @@ -656,7 +656,7 @@ static void __init of_parse_and_init_cpus(void)
>  			if (bootcpu_valid) {
>  				pr_err("%pOF: duplicate boot cpu reg property in DT\n",
>  					dn);
> -				goto next;
> +				continue;
>  			}
>  

People get very upset when CPU numbering changes, so I'd prefer to avoid
this if possible.

Mark.

>  			bootcpu_valid = true;
> -- 
> 2.34.1
>