[PATCH] blk-throttle: fix divide-by-zero on legacy iops limit of 0

[Tao Cui]

From: Tao Cui <cuitao@xxxxxxxxxx>

Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1
throttle iops file (blkio.throttle.{read,write}_iops_device) silently
truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into
an unsigned int field with no clamping. The cgroup v2 path,
tg_set_limit(), already clamps the same kind of value with
min_t(u64, val, UINT_MAX), but the legacy path never did. Note that
the "!v -> U64_MAX" mapping only catches an explicit zero and does not
catch a value that truncates to zero.

With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and
the next IO reaches tg_within_iops_limit(), which computes

    jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);

triggering a divide-by-zero oops.

Fix it in two places:

  * tg_set_conf(): clamp the value to UINT_MAX, consistent with
    tg_set_limit(). This closes the truncation root cause (and the
    general silent truncation for any value above UINT_MAX).

  * tg_dispatch_iops_time(): treat iops_limit == 0 as unlimited so the
    divide in tg_within_iops_limit() is never reached, defending
    against any future path that could produce a zero limit.

Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
---
 block/blk-throttle.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index ffc3b70065d4..3f3c1374f4b2 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *bi
 	u32 iops_limit = tg_iops_limit(tg, rw);
 	unsigned long iops_wait;
 
-	if (iops_limit == UINT_MAX || tg->flags & THROTL_TG_CANCELING)
+	/*
+	 * iops_limit == 0 is not a valid limit. Treat it as unlimited so we
+	 * never reach the HZ / iops_limit divide in tg_within_iops_limit().
+	 */
+	if (iops_limit == UINT_MAX || iops_limit == 0 ||
+	    tg->flags & THROTL_TG_CANCELING)
 		return 0;
 
 	tg_update_slice(tg, rw);
@@ -1386,7 +1391,8 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
 	if (is_u64)
 		*(u64 *)((void *)tg + of_cft(of)->private) = v;
 	else
-		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
+		*(unsigned int *)((void *)tg + of_cft(of)->private) =
+			min_t(u64, v, UINT_MAX);
 
 	tg_conf_updated(tg, false);
 	ret = 0;
-- 
2.43.0