Re: [PATCH 10/19] mm/damon/core: validate params for probe hits weighted sum overflow

[SJ Park <sj@kernel.org>]

On Fri, 10 Jul 2026 14:07:43 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] Pre-existing integer overflow in `damon_merge_two_regions()` during weighted average calculations.
> - [High] Pre-existing integer overflows in `damon_sample_bp_to_count()` and `damon_sample_count_to_bp()`.
> --
> 
> --- Patch [10]: [PATCH 10/19] mm/damon/core: validate params for probe hits weighted sum overflow ---
> commit 74455514643a15cd1b70c8a9afa3237ba4226d34
> Author: SJ Park <sj@kernel.org>
> Date:   Fri Jul 10 13:46:39 2026 +0000
> 
>     mm/damon/core: validate params for probe hits weighted sum overflow
> 
> This commit adds parameter validation to prevent integer overflows in probe
> hits weighted sum calculations. It ensures that user-configured probe weights
> do not cause the weighted sum of probe hits to exceed the maximum possible
> value during monitoring.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 4f1425e56950b..a0e6a5129fbf3 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -1335,6 +1335,9 @@ static void damos_set_filters_default_reject(struct damos *s)
> >  static bool damon_valid_probe_params(struct damon_ctx *ctx)
> >  {
> >  	unsigned long sample_interval;
> > +	unsigned char max_probe_hits;
> > +	struct damon_probe *probe;
> > +	unsigned int wsum, wsum_to_add;
> >  
> >  	if (!damon_has_probe_weights(ctx))
> >  		return true;
> > @@ -1342,6 +1345,18 @@ static bool damon_valid_probe_params(struct damon_ctx *ctx)
> >  	sample_interval = ctx->attrs.sample_interval ? : 1;
> >  	if (ctx->attrs.aggr_interval / sample_interval > U8_MAX)
> >  		return false;
> > +
> > +	/* invalid if probe hits weighted sum can overflow */
> > +	max_probe_hits = damon_nr_samples_per_aggr(&ctx->attrs);
> > +	wsum = 0;
> > +	damon_for_each_probe(probe, ctx) {
> > +		if (probe->weight > UINT_MAX / max_probe_hits)
> > +			return false;
> > +		wsum_to_add = probe->weight * max_probe_hits;
> > +		if (UINT_MAX - wsum < wsum_to_add)
> > +			return false;
> > +		wsum += wsum_to_add;
> > +	}
> >  	return true;
> >  }
> >  
> 
> [Severity: High]
> This is a pre-existing issue, but does damon_merge_two_regions() suffer from an
> unsigned long overflow during average calculations?
> 
> mm/damon/core.c:damon_merge_two_regions() {
> 	...
> 	l->nr_accesses = (l->nr_accesses * sz_l + r->nr_accesses * sz_r) /
> 			(sz_l + sz_r);
> 	l->age = (l->age * sz_l + r->age * sz_r) / (sz_l + sz_r);
> 	...
> }
> 
> During the calculation of the merged region metrics, if a region size reaches
> 1GB and nr_accesses is just 4, their product is 4,294,967,296. On 32-bit
> architectures where unsigned long is 32 bits, this will instantly overflow.
> 
> Additionally, on 64-bit architectures, the age variable could grow indefinitely
> and overflow if a region becomes very large over a long uptime. Can this lead
> to corrupted statistics and misfiring of memory management actions?

Known low priority issue.

> 
> 
> [Severity: High]
> This is a pre-existing issue, but I noticed a potential integer overflow in
> damon_sample_bp_to_count() and damon_sample_count_to_bp():
> 
> mm/damon/core.c:damon_sample_bp_to_count() {
> 	return bp * damon_nr_samples_per_aggr(attrs) / 10000;
> }
> 
> mm/damon/core.c:damon_sample_count_to_bp() {
> 	return mult_frac(count, 10000, damon_nr_samples_per_aggr(attrs));
> }
> 
> Can these calculations overflow? The functions perform 32-bit multiplication of
> bp (or the remainder in mult_frac) and the max samples using unsigned int
> arithmetic.
> 
> If max_samples (aggr_interval / sample_interval) exceeds 429,496, which is
> possible with a high aggregation interval compared to the sampling interval
> (e.g., 5s aggregation and 10us sample intervals), the result could exceed
> UINT_MAX (4,294,967,295) and silently overflow. Could this corrupt monitoring
> region access counts during dynamic DAMON attribute updates?

Known low priority issue.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260710134651.18084-1-sj@kernel.org?part=10


Thanks,
SJ