[PATCH 6.19 057/311] net/sched: sch_hfsc: fix divide-by-zero in rtsc_min()

Greg Kroah-Hartman
2026-04-08T18:52:15Z

From: Xiang Mei <xmei5@asu.edu>

[ Upstream commit 4576100b8cd03118267513cafacde164b498b322 ]

m2sm() converts a u32 slope to a u64 scaled value. For large inputs
(e.g. m1=4000000000), the result can reach 2^32. rtsc_min() stores
the difference of two such u64 values in a u32 variable `dsm` and
uses it as a divisor. When the difference is exactly 2^32 the
truncation yields zero, causing a divide-by-zero oops in the
concave-curve intersection path:

Oops: divide error: 0000
RIP: 0010:rtsc_min (net/sched/sch_hfsc.c:601)
Call Trace:
init_ed (net/sched/sch_hfsc.c:629)
hfsc_enqueue (net/sched/sch_hfsc.c:1569)
[...]

Widen `dsm` to u64 and replace do_div() with div64_u64() so the full
difference is preserved.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260326204310.1549327-1-xmei5@asu.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/sched/sch_hfsc.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index d8fd35da32a7c..57221522fe56d 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -555,7 +555,7 @@ static void
rtsc_min(struct runtime_sc *rtsc, struct internal_sc *isc, u64 x, u64 y)
{
u64 y1, y2, dx, dy;
- u32 dsm;
+ u64 dsm;

if (isc->sm1 <= isc->sm2) {
/* service curve is convex */
@@ -598,7 +598,7 @@ rtsc_min(struct runtime_sc *rtsc, struct internal_sc *isc, u64 x, u64 y)
*/
dx = (y1 - y) << SM_SHIFT;
dsm = isc->sm1 - isc->sm2;
- do_div(dx, dsm);
+ dx = div64_u64(dx, dsm);
/*
* check if (x, y1) belongs to the 1st segment of rtsc.
* if so, add the offset.
--
2.53.0