[RFC PATCH v2 2/2] mm/damon/lru_sort: error out for >10000 active_mem_bp

[SJ Park <sj@kernel.org>]

damos_quota_score() can trigger division by zero if the target value is
zero.  DAMON_LRU_SORT lets users set the target value for the hot memory
scheme via active_mem_bp parameter.  It avoids setting it as the target
value if the parameter value is zero.  However, it also sets the cold
memory scheme with a target value that is calculated as '10000 -
active_mem_bp + 2'.  Hence, if a user sets active_mem_bp 10002, the cold
memory scheme's quota goal target value can be zero.  As a result,
division by zero can be triggered.  Fix by returning an error when the
user tries to start DAMON with >10000 active_mem_bp parameter value.

It makes no sense to set active_mem_bp with 10002.  It also requires
module parameters write permission to reproduce the issue.  That said,
the consequence is quite bad.

One reliable way to reproduce the issue is like below:

    # cd /sys/module/damon_lru_sort/parameters
    # echo 1000 > wmarks_high
    # echo 995 > wmarks_mid
    # echo 0 > wmarks_low
    # echo 10002 > active_mem_bp
    # echo Y > enabled
    # dmesg -w
    [...]
    [  597.421247] Oops: divide error: 0000 [#1] SMP NOPTI
    [  597.428848] RIP: 0010:damos_quota_score+0x6f/0x480

This issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260801213028.5127-1-sj@kernel.org

Fixes: 40d98d31cd70 ("mm/damon/lru_sort: support active:inactive memory ratio based auto-tuning")
Cc: <stable@vger.kernel.org> # 7.0.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/lru_sort.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index 7e077084cb03a..3920d5042f109 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -233,6 +233,8 @@ static int damon_lru_sort_add_quota_goals(struct damos *hot_scheme,
 
 	if (!active_mem_bp)
 		return 0;
+	if (10000 < active_mem_bp)
+		return -EINVAL;
 	goal = damos_new_quota_goal(DAMOS_QUOTA_ACTIVE_MEM_BP, active_mem_bp);
 	if (!goal)
 		return -ENOMEM;
-- 
2.47.3