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

[SeongJae Park <sj@kernel.org>]

On Wed, 25 Mar 2026 07:37:39 -0700 SeongJae Park <sj@kernel.org> wrote:

> Forwarding Sashiko review for doing discussions via mails.
> 
> # review url: https://sashiko.dev/#/patchset/20260325073034.140353-1-objecting@objecting.org
> # start of sashiko.dev inline review
> commit 7e17cbe1dc95a0cd1a1410be38ab6964f78fb15c
> Author: Josh Law <objecting@objecting.org>
> 
> 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.

I think sashiko's comments make sense.  I also suggested using
node_state(N_MEMORY).  What do you think, Josh?


Thanks,
SJ

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