net/sched: sch_hfsc: fix divide-by-zero in rtsc_min()

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: 1da177e ("Linux-2.6.12-rc2")
Reported-by: Weiming Shi <[email protected]>
Signed-off-by: Xiang Mei <[email protected]>
Acked-by: Jamal Hadi Salim <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: n132

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.