Re: (sashiko review) [PATCH] mm/damon/core: validate goal nid before accessing node data

[SeongJae Park]

Forwarding Sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/20260325073034.140353-1-objecting@xxxxxxxxxxxxx
# start of sashiko.dev inline review
commit 7e17cbe1dc95a0cd1a1410be38ab6964f78fb15c
Author: Josh Law <objecting@xxxxxxxxxxxxx>

mm/damon/core: validate goal nid before accessing node data

This commit adds bounds checking and node_online() validation for the
goal->nid in the DAMON quota metrics. It prevents out-of-bounds accesses
to NODE_DATA() when an invalid or offline NUMA node is provided by userspace.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index db6c67e52d2b..81ce27baa1a1 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2245,6 +2245,10 @@ static __kernel_ulong_t damos_get_node_mem_bp(
>  	struct sysinfo i;
>  	__kernel_ulong_t numerator;
>  
> +	if (goal->nid < 0 || goal->nid >= MAX_NUMNODES ||
> +	    !node_online(goal->nid))
> +		return 0;
> +
>  	si_meminfo_node(&i, goal->nid);
>  	if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP)
>  		numerator = i.totalram - i.freeram;
>  	else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
>  		numerator = i.freeram;
>  	return mult_frac(numerator, 10000, i.totalram);

Could this cause a division by zero if the node is online but memoryless?

For CPU-only compute nodes or nodes where all memory has been hot-removed,
node_online(goal->nid) will evaluate to true. When si_meminfo_node() is
called for these memoryless nodes, it populates the sysinfo struct with
i.totalram = 0.

The mult_frac() macro unconditionally performs integer division and modulo
by the third argument, which could trigger a kernel panic when dividing by
zero.

Would it be safer to check if i.totalram is zero before returning, or use
node_state(goal->nid, N_MEMORY) instead of node_online()?

[ ... ]

> @@ -2261,6 +2265,10 @@ static unsigned long damos_get_node_memcg_used_bp(
>  	unsigned long used_pages, numerator;
>  	struct sysinfo i;
>  
> +	if (goal->nid < 0 || goal->nid >= MAX_NUMNODES ||
> +	    !node_online(goal->nid))
> +		return 0;
> +
>  	memcg = mem_cgroup_get_from_id(goal->memcg_id);

The same division by zero risk exists here when mult_frac() is called at the
end of this function.


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