[PATCH] mm/damon: reject zero sampling interval

[Karthikeyan KS <karthiproffesional@gmail.com>]

damon_set_attrs() already rejects inverted intervals (sample_interval >
aggr_interval) and invalid region counts, but it accepts
sample_interval == 0.

A zero sampling interval is not a valid monitoring period.  Sysfs "on"
and "commit" both install attrs via damon_set_attrs(), so kdamond then
calls kdamond_usleep(0) and busy-spins.

Return -EINVAL if sample_interval is zero.

Signed-off-by: Karthikeyan KS <karthiproffesional@gmail.com>
---

Hello,

I recently started reading DAMON and just found this one.  This
change only tightens damon_set_attrs() validation, in the same place
as the existing sample > aggr and min_nr_regions checks.  It is not a
crash, UAF, or race.

sample_us=0 is accepted today, so kdamond_usleep(0) busy-spins (about
one core).  Sysfs "on" and "commit" both go through damon_set_attrs();
one check covers both.

Checked on v7.3-rc3 (238650ef6c7c):

  unpatched: echo on with sample_us=0 succeeds; kdamond uses ~97% of
             one core.  echo commit of 0 onto a running kdamond also
             succeeds and it starts spinning.

  patched:   both on and commit return -EINVAL.  Valid sample_us=5000
             still starts.  After a rejected commit the same pid stays
             on and does not spin.

aggr_us=0 with a positive sample_us is already -EINVAL from the
ordering check.

Thanks,

 mm/damon/core.c             |  2 ++
 mm/damon/tests/core-kunit.h | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 644daf5a1656..376c3e2cbe00 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1060,6 +1060,8 @@ int damon_set_attrs(struct damon_ctx *ctx, struct damon_attrs *attrs)
 		return -EINVAL;
 	if (attrs->min_nr_regions > attrs->max_nr_regions)
 		return -EINVAL;
+	if (!attrs->sample_interval)
+		return -EINVAL;
 	if (attrs->sample_interval > attrs->aggr_interval)
 		return -EINVAL;
 
diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 4a536d41cdb2..ec8ef71956c8 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -658,6 +658,19 @@ static void damon_test_set_attrs(struct kunit *test)
 	invalid_attrs.aggr_interval = 4999;
 	KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
 
+	invalid_attrs = valid_attrs;
+	invalid_attrs.sample_interval = 0;
+	KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
+
+	invalid_attrs = valid_attrs;
+	invalid_attrs.aggr_interval = 0;
+	KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
+
+	invalid_attrs = valid_attrs;
+	invalid_attrs.sample_interval = 0;
+	invalid_attrs.aggr_interval = 0;
+	KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
+
 	damon_destroy_ctx(c);
 }

-- 
2.34.1