Re: [PATCH] mm/damon/core: validate ranges in damon_set_regions()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The introduced validation logic in `damon_set_regions()` is flawed and fails to prevent the `WARN_ONCE` and divide-by-zero crashes it intends to fix. Distinct valid regions within the same alignment block bypass the check, leading to duplicated overlapping regions, inverted hole regions, and a kernel panic.
--

commit 79a8d61a7b1112d3a6da1cff84b7f4a28c66e465
Author: SJ Park <sj@kernel.org>

mm/damon/core: validate ranges in damon_set_regions()

This commit attempts to fix a zero-length region issue that could lead to
a WARN_ONCE and a divide-by-zero crash in DAMON. It adds a validation
loop in damon_set_regions() to reject empty or invalid target regions
before they are processed.

Fixes: 43b0536cb471 ("mm/damon: introduce DAMON-based Reclamation (DAMON_RECLAIM)")
Cc: <stable@vger.kernel.org> # 5.16.x

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 027250e43c66f..3dd2750c2ef20 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -356,6 +356,12 @@ int damon_set_regions(struct damon_target *t, struct damon_addr_range *ranges,
>  	unsigned int i;
>  	int err;
>  
> +	for (i = 0; i < nr_ranges; i++) {
> +		if (ALIGN_DOWN(ranges[i].start, min_region_sz) >=
> +				ALIGN(ranges[i].end, min_region_sz))
> +			return -EINVAL;
> +	}
> +

[Severity: High]
This is a pre-existing issue, but does this validation fully prevent the
divide-by-zero crash?

If a user provides two unaligned regions that fall within the same
min_region_sz block (for example, [10, 20] and [20, 30]), this check will
evaluate ALIGN_DOWN(10) >= ALIGN(20), which becomes 0 >= 4096. This allows
them to bypass rejection.

Because they pass, damon_set_regions() inflates both regions to [0, 4096],
creating identical overlapping regions in the list.

If a subsequent update covers these regions (like [0, 8192]), the
intersecting boundaries are adjusted but first->ar.end is left at 4096 and
last->ar.start at 0. This leads to damon_fill_regions_holes() creating an
inverted hole region:

mm/damon/core.c:damon_fill_regions_holes() {
    ...
		if (r->ar.end != next->ar.start) {
			newr = damon_new_region(r->ar.end, next->ar.start);
    ...
}

Calling damon_new_region(4096, 0) triggers the WARN_ONCE(start >= end) and
results in a region with a negative size (ULONG_MAX - 4095).

When damon_merge_two_regions() eventually tries to merge [0, 4096] with
[4096, 0], the combined size underflows to exactly 0, causing a panic here:

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);
    ...
}

Should the alignment check be stricter, or does damon_set_regions() need
additional logic to merge or reject identically aligned overlapping blocks?

>  	/* Remove regions which are not in the new ranges */
>  	damon_for_each_region_safe(r, next, t) {
>  		for (i = 0; i < nr_ranges; i++) {

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260630035221.146458-1-sj@kernel.org?part=1