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

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

[SeongJae Park]

DAMON core logic assumes zero length regions don't exist.  However, a
few DAMON API callers including DAMON_SYSFS, DAMON_RECLAIM and
DAMON_LRU_SORT allow users to set empty monitoring target regions.  This
could result in WARN_ONCE() on CONFIG_DAMON_DEBUG_SANITY enabled kernel,
and divide-by-zero from damon_merge_two_regions().

For example, the WANR_ONCE() can be triggered like below.

    # grep DAMON_DEBUG_SANITY /boot/config-$(uname -r)
    # CONFIG_DAMON_DEBUG_SANITY=y
    # damo start
    # cd /sys/kernel/mm/damon/admin/kdamonds/0
    # echo 0 > contexts/0/targets/0/regions/0/start
    # echo 0 > contexts/0/targets/0/regions/0/end
    # echo commit > state
    # dmesg
    [....]
    [   73.705780] ------------[ cut here ]------------
    [   73.707552] start 0 >= end 0
    [   73.708452] WARNING: mm/damon/core.c:359 at damon_new_region+0x6e/0x80, CPU#1: kdamond.0/758
    [...]

Disallow empty region user inputs by updating the validation logic.

Fixes: 43b0536cb471 ("mm/damon: introduce DAMON-based Reclamation (DAMON_RECLAIM)")
Cc: <stable@vger.kernel.org> # 5.16.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 7e4b9affc5b06..b3100d7fa5596 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -358,6 +358,11 @@ 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 (ranges[i].start >= ranges[i].end)
+			return -EINVAL;
+	}
+
 	/* Remove regions which are not in the new ranges */
 	damon_for_each_region_safe(r, next, t) {
 		for (i = 0; i < nr_ranges; i++) {

base-commit: f5cde2d41633df3fb1965efa00bfa827ca41836c
-- 
2.47.3

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

[SeongJae Park]

On Sat, 27 Jun 2026 10:00:56 -0700 SeongJae Park <sj@kernel.org> wrote:

> DAMON core logic assumes zero length regions don't exist.  However, a
> few DAMON API callers including DAMON_SYSFS, DAMON_RECLAIM and
> DAMON_LRU_SORT allow users to set empty monitoring target regions.  This
> could result in WARN_ONCE() on CONFIG_DAMON_DEBUG_SANITY enabled kernel,
> and divide-by-zero from damon_merge_two_regions().
> 
> For example, the WANR_ONCE() can be triggered like below.
> 
>     # grep DAMON_DEBUG_SANITY /boot/config-$(uname -r)
>     # CONFIG_DAMON_DEBUG_SANITY=y
>     # damo start
>     # cd /sys/kernel/mm/damon/admin/kdamonds/0
>     # echo 0 > contexts/0/targets/0/regions/0/start
>     # echo 0 > contexts/0/targets/0/regions/0/end
>     # echo commit > state
>     # dmesg
>     [....]
>     [   73.705780] ------------[ cut here ]------------
>     [   73.707552] start 0 >= end 0
>     [   73.708452] WARNING: mm/damon/core.c:359 at damon_new_region+0x6e/0x80, CPU#1: kdamond.0/758
>     [...]
> 
> Disallow empty region user inputs by updating the validation logic.

The above description is wrong, since this is not updating an existing
validation but adding a new validation.

> 
> Fixes: 43b0536cb471 ("mm/damon: introduce DAMON-based Reclamation (DAMON_RECLAIM)")
> Cc: <stable@vger.kernel.org> # 5.16.x
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  mm/damon/core.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 7e4b9affc5b06..b3100d7fa5596 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -358,6 +358,11 @@ 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 (ranges[i].start >= ranges[i].end)
> +			return -EINVAL;
> +	}
> +

Sashiko found [1] this is not complete, since eventually this function uses
aligned addresses.  I will address that in the next revision by doing the
validation with the aligned addresses.

[1] https://lore.kernel.org/20260627172406.3794-1-sj@kernel.org


Thanks,
SJ

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