From e16b8d640ec99b28bc827560edcf9706e610c3aa Mon Sep 17 00:00:00 2001 From: SJ Park Date: Mon, 3 Aug 2026 06:40:32 -0700 Subject: [PATCH] samples/damon/mtier: error out for zero quota goal target values commit a16fd3ad9d89b05475864da97327870464611736 upstream. Patch series "mm/damon: avoid division by zero from damos_quota_score()". DAMON_SAMPLE_MTIER and DAMON_LRU_SORT allow the user to trigger division by zero in damos_quota_score(). Avoid it by adding parameters validation checks. This patch (of 2): damos_quota_score() can trigger division by zero if the target_value is zero. DAMON_SAMPLE_MTIER lets users set the target_value via node0_mem_{used,free}_bp parameters. It doesn't guard zero value case, though. As a result, users can trigger division by zero. Fix the issue by returning an error when the user tries to start DAMON with zero node0_mem_{used,free}_bp parameter values. DAMON_SAMPLE_MTIER is just a sample module, but the consequence is quite bad. Also the zero node0_mem_free_bp parameter might look like a reasonable setup to some users. Hence, the issue might really happen in the real world. One reliable way to reproduce the issue is like below: # cd /sys/module/damon_sample_mtier/parameters # echo 4096 > node0_start_addr # echo 8192 > node0_end_addr # echo 8192 > node1_start_addr # echo 81920 > node1_end_addr # echo 0 > node0_mem_free_bp # echo Y > enabled # dmesg -w [...] [18792.235916] Oops: divide error: 0000 [#1] SMP NOPTI [...] [18792.242787] RIP: 0010:damos_quota_score+0x6f/0x480 [...] This issue was discovered [1] by Sashiko. Link: https://lore.kernel.org/20260803134034.15217-1-sj@kernel.org Link: https://lore.kernel.org/20260803134034.15217-2-sj@kernel.org Link: https://lore.kernel.org/20260801202657.117135-1-sj@kernel.org [1] Fixes: c5e67d40a102 ("samples/damon/mtier: add parameters for node0 memory usage") Signed-off-by: SJ Park Cc: # 6.17.x Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- samples/damon/mtier.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c index dab8fb10b617..3dae16fd2e11 100644 --- a/samples/damon/mtier.c +++ b/samples/damon/mtier.c @@ -153,6 +153,9 @@ static struct damon_ctx *damon_sample_mtier_build_ctx(bool promote) if (!scheme) goto free_out; damon_set_schemes(ctx, &scheme, 1); + /* zero target value causes division by zero in damos_quota_store() */ + if (!node0_mem_used_bp || !node0_mem_free_bp) + goto free_out; quota_goal = damos_new_quota_goal( promote ? DAMOS_QUOTA_NODE_MEM_USED_BP : DAMOS_QUOTA_NODE_MEM_FREE_BP,