[RFC PATCH 6.18.y 3/5] mm/damon/sysfs-schemes: read sysfs_filter->sz_range only once

[SJ Park <sj@kernel.org>]

DAMON sysfs interface reads the user-provided size range arguments for
hugepage_size type DAMOS filter twice.  Once for validation, and once
again for assignments to the variable that will be passed to the core
layer.  If the user updates the arguments in parallel, an invalid size
range could be passed to the core layer.  Avoid it by doing the
assignments first, and then validating the assigned variables before
passing those to the core layer.

User impact of the bug should be trivial.  From the core layer's
perspective, the invalid size range is not really invalid.  It just works
as having a weird size range.  No critical issues such as a crash or a
leak could happen.  And sane users ain't do such parallel arguments update
anyway.  If they do, such racy behavior is arguably somewhat expected and
deserved.  That said, there is no reason to keep such races.

Link: https://lore.kernel.org/20260914142327.92510-3-sj@kernel.org
Fixes: ea1f204ba29a ("mm/damon/sysfs-schemes: add files for setting damos_filter->sz_range")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@davidgow.net>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: <stable@vger.kernel.org>
(cherry picked from commit af19aff38d6faea35b10f26cca32d0e54f8aea5e)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/sysfs-schemes.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 37e0a8bef4b1f..0f945ef043709 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/mm/damon/sysfs-schemes.c
@@ -2493,12 +2493,11 @@ static int damon_sysfs_add_scheme_filters(struct damos *scheme,
 		} else if (filter->type == DAMOS_FILTER_TYPE_TARGET) {
 			filter->target_idx = sysfs_filter->target_idx;
 		} else if (filter->type == DAMOS_FILTER_TYPE_HUGEPAGE_SIZE) {
-			if (sysfs_filter->sz_range.min >
-					sysfs_filter->sz_range.max) {
+			filter->sz_range = sysfs_filter->sz_range;
+			if (filter->sz_range.min > filter->sz_range.max) {
 				damos_destroy_filter(filter);
 				return -EINVAL;
 			}
-			filter->sz_range = sysfs_filter->sz_range;
 		}
 
 		damos_add_filter(scheme, filter);
-- 
2.47.3