[PATCH 6.18.y] mm/damon/sysfs: read addr_unit only once in damon_sysfs_apply_inputs()

[PATCH 6.18.y] mm/damon/sysfs: read addr_unit only once in damon_sysfs_apply_inputs()

[SJ Park]

damon_sysfs_apply_inputs() reads addr_unit twice.  It could race with
addr_unit_store().  As a result, the min_region_sz could wrongly be set
up.  Read it once.

The user impact is trivial.  Sane users ain't update the parameter in
parallel.  Even if it happens, the DAMON core layer handles the wrong
min_region_sz (!is_power_of_2()).  Even if somehow the race ended up
making a min_region_sz that is different from the user's intention but
still valid, only monitoring itself runs differently than expected.  No
critical consequences like kernel panic or memory corruption happen.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260715031002.108504-6-sj@kernel.org
Link: https://lore.kernel.org/20260714142950.100711-1-sj@kernel.org [1]
Fixes: 540a2aebc657 ("mm/damon/sysfs: implement addr_unit file under context dir")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.18.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit ab4d9358e32316fa39b5f1f5360292135978c3d9)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index a6c3e83a3dad5..0b14e342ccf61 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1437,11 +1437,11 @@ static int damon_sysfs_apply_inputs(struct damon_ctx *ctx,
 	err = damon_select_ops(ctx, sys_ctx->ops_id);
 	if (err)
 		return err;
-	ctx->addr_unit = sys_ctx->addr_unit;
+	ctx->addr_unit = READ_ONCE(sys_ctx->addr_unit);
 	/* addr_unit is respected by only DAMON_OPS_PADDR */
 	if (sys_ctx->ops_id == DAMON_OPS_PADDR)
 		ctx->min_sz_region = max(
-				DAMON_MIN_REGION / sys_ctx->addr_unit, 1);
+				DAMON_MIN_REGION / ctx->addr_unit, 1);
 	err = damon_sysfs_set_attrs(ctx, sys_ctx->attrs);
 	if (err)
 		return err;
-- 
2.47.3

Re: [PATCH 6.18.y] mm/damon/sysfs: read addr_unit only once in damon_sysfs_apply_inputs()

[sashiko-bot]

> damon_sysfs_apply_inputs() reads addr_unit twice.  It could race with
> addr_unit_store().  As a result, the min_region_sz could wrongly be set
> up.  Read it once.
> 
> The user impact is trivial.  Sane users ain't update the parameter in
> parallel.  Even if it happens, the DAMON core layer handles the wrong
> min_region_sz (!is_power_of_2()).  Even if somehow the race ended up
> making a min_region_sz that is different from the user's intention but
> still valid, only monitoring itself runs differently than expected.  No
> critical consequences like kernel panic or memory corruption happen.
> 
> The issue was discovered [1] by Sashiko.
> 
> Link: https://lore.kernel.org/20260715031002.108504-6-sj@kernel.org
> Link: https://lore.kernel.org/20260714142950.100711-1-sj@kernel.org [1]
> Fixes: 540a2aebc657 ("mm/damon/sysfs: implement addr_unit file under context dir")
> Signed-off-by: SJ Park <sj@kernel.org>
> Cc: <stable@vger.kernel.org> # 6.18.x
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit ab4d9358e32316fa39b5f1f5360292135978c3d9)
> Signed-off-by: SJ Park <sj@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909032452.118744-1-sj@kernel.org?part=1