netem: fix zero division in tabledist

Currently it is possible to craft a special netlink RTM_NEWQDISC
command that can result in jitter being equal to 0x80000000. It is
enough to set the 32 bit jitter to 0x02000000 (it will later be
multiplied by 2^6) or just set the 64 bit jitter via
TCA_NETEM_JITTER64. This causes an overflow during the generation of
uniformly distributed numbers in tabledist(), which in turn leads to
division by zero (sigma != 0, but sigma * 2 is 0).

The related fragment of code needs 32-bit division - see commit
9b0ed89 ("netem: remove unnecessary 64 bit modulus"), so switching to
64 bit is not an option.

Fix the issue by keeping the value of jitter within the range that can
be adequately handled by tabledist() - [0;INT_MAX]. As negative std
deviation makes no sense, take the absolute value of the passed value
and cap it at INT_MAX. Inside tabledist(), switch to unsigned 32 bit
arithmetic in order to prevent overflows.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Reported-by: syzbot+ec762a6342ad0d3c0d8f@syzkaller.appspotmail.com
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Link: https://lore.kernel.org/r/20201028170731.1383332-1-aleksandrnogikh@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_netem.c

@@ -330,7 +330,7 @@ static s64 tabledist(s64 mu, s32 sigma,
 
 	/* default uniform distribution */
 	if (dist == NULL)
-		return ((rnd % (2 * sigma)) + mu) - sigma;
+		return ((rnd % (2 * (u32)sigma)) + mu) - sigma;
 
 	t = dist->table[rnd % dist->size];
 	x = (sigma % NETEM_DIST_SCALE) * t;
@@ -812,6 +812,10 @@ static void get_slot(struct netem_sched_data *q, const struct nlattr *attr)
 		q->slot_config.max_packets = INT_MAX;
 	if (q->slot_config.max_bytes == 0)
 		q->slot_config.max_bytes = INT_MAX;
+
+	/* capping dist_jitter to the range acceptable by tabledist() */
+	q->slot_config.dist_jitter = min_t(__s64, INT_MAX, abs(q->slot_config.dist_jitter));
+
 	q->slot.packets_left = q->slot_config.max_packets;
 	q->slot.bytes_left = q->slot_config.max_bytes;
 	if (q->slot_config.min_delay | q->slot_config.max_delay |
@@ -1037,6 +1041,9 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 	if (tb[TCA_NETEM_SLOT])
 		get_slot(q, tb[TCA_NETEM_SLOT]);
 
+	/* capping jitter to the range acceptable by tabledist() */
+	q->jitter = min_t(s64, abs(q->jitter), INT_MAX);
+
 	return ret;
 
 get_table_failure: