samples/damon/mtier: error out for zero quota goal target values

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
Fixes: c5e67d40a102 ("samples/damon/mtier: add parameters for node0 memory usage")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.17.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
index 3785b0c7ffb17571d41567d0d898fd19e988272f..dae6929f827c28ef7f8e7c2e2fa8f33c3c659668 100644 (file)
--- a/samples/damon/mtier.c
+++ b/samples/damon/mtier.c
@@ -156,6 +156,9 @@ 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,