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

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Date: Sat, 22 Jun 2024 16:31:14 +0800
From: Lingyue <lingyue@...omi.com>
To: Mark Rutland <mark.rutland@....com>
CC: <catalin.marinas@....com>, <will@...nel.org>, <dianders@...omium.org>,
	<swboyd@...omium.org>, <frederic@...nel.org>, <james.morse@....com>,
	<scott@...amperecomputing.com>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <huangshaobo3@...omi.com>,
	<huangjun7@...omi.com>
Subject: Re: [PATCH] arm64: smp: do not allocate CPU IDs to invalid CPU nodes

On Fri, Jun 21, 2024 at 11:12:06AM +0100, Mark Rutland wrote:
> 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.
>
Yes, I also don't find any documented requirement about it.

> > 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.
>
In this case, of course we can modify the arch topology code to solve the problem.
However, I propose that if we can ensure there are no gaps in the cpu_possible_mask,
we can solve such misuse issues once and for all, without having to dig and fix other
potential similar problems one by one.

> > 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@...omi.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.
> 
This modification will only affect CPU ID allocation if there are invalid or
duplicate CPU nodes in device tree. IMO, it is not a typical use case, but
please let me know if there are any other use case.

Many thanks for your response.

Lingyue.

> >  			bootcpu_valid = true;
> > -- 
> > 2.34.1
> >