[PATCH 7.1 194/271] mm/damon/lru_sort: error out for >10000 active_mem_bp

7.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: SJ Park <[email protected]>

commit 06befa61c427e74319781e6f35a364cfc32dbae8 upstream.

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.

Link: https://lore.kernel.org/[email protected]
Link: https://lore.kernel.org/[email protected] [1]
Fixes: 40d98d31cd70 ("mm/damon/lru_sort: support active:inactive memory ratio based auto-tuning")
Signed-off-by: SJ Park <[email protected]>
Cc: <[email protected]> # 7.0.x
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 mm/damon/lru_sort.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -234,6 +234,8 @@ static int damon_lru_sort_add_quota_goal
 
 	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;