[RFC PATCH v1.1] mm/damon/core: validate ranges in damon_set_regions()

[SJ Park <sj@kernel.org>]

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
    [...]

All DAMON API callers eventually use damon_set_regions() to setup the
regions.  Add the validation logic in the function.

Fixes: 43b0536cb471 ("mm/damon: introduce DAMON-based Reclamation (DAMON_RECLAIM)")
Cc: <stable@vger.kernel.org> # 5.16.x
Signed-off-by: SJ Park <sj@kernel.org>
---
Changes from RFC v1
- RFC v1: https://lore.kernel.org/20260627170057.1867-1-sj@kernel.org
- Fixup the commit message for how the fix is made.
- Do the validation with min_region_sz-aligned addresses.

FYI, this fix cannot be applied as is to the commit that introduced this
class of bugs, because damon_set_regions() was introduced after the bug.
I considered making three fixes for each caller to make the backporting
on the old kernels easy.  However, the first LTS kernel having the bug
is 6.1.y, which has damon_set_regions() and all the callers are using
it.  So porting this to necessary stable kernels should be easy enough.

 mm/damon/core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index d99f7a297fdde..df0cc699494fe 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_DOWN(ranges[i].end, min_region_sz))
+			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: 7c001190b88a32f80c93a6ac302af59a9756309c
-- 
2.47.3