From: Aleksandr Nogikh nogikh@google.com
[ Upstream commit eadd1befdd778a1eca57fad058782bd22b4db804 ]
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: 1da177e4c3f4 ("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.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/sched/sch_netem.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
--- a/net/sched/sch_netem.c +++ b/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_ 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 *sc 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: