prb_calc_retire_blk_tmo() computes in 32-bit int arithmetic: mbits = (blk_size_in_bytes * 8) / (1024 * 1024); If I'm reading the validation right, tp_block_size is user controlled and packet_set_ring() only rejects values that are <= 0 as int or not page aligned, so a 256MiB block goes right through (and alloc_one_pg_vec_page() even has a vzalloc fallback for it). 0x10000000 * 8 wraps to INT_MIN, and on a NIC reporting 1 Gbps (div == 1) the function ends up returning -2047. The condition is actually (8 * size) mod 2^32 >= 2^31 && div == 1, so the trigger set is [256,512), [768,1024), [1280,1536) and [1792,2048) MiB. Other sizes wrap to non-negative values and faster links divide the unsigned value back below 2^31, which is why this doesn't blow up for everyone. What makes it fatal is what happens next in init_prb_bdqc(): p1->interval_ktime = ms_to_ktime(prb_calc_retire_blk_tmo(...)); hrtimer_start(&p1->retire_blk_timer, p1->interval_ktime, HRTIMER_MODE_REL_SOFT); A negative relative timeout expires immediately. The callback unconditionally returns HRTIMER_RESTART, and hrtimer_forward() turns the negative interval into hrtimer_resolution: if (interval < hrtimer_resolution) interval = hrtimer_resolution; So the SOFT timer re-fires at the maximum rate forever, holding sk_receive_queue.lock each pass. One CPU spins in softirq until the socket is closed. Repeat with more rings and the machine is gone. This is mostly a new problem: the overflow is ancient, but with msecs_to_jiffies() a negative timeout just got clamped to a huge unsigned value, i.e. the retire timer effectively never fired. f7460d2989fa ("net: af_packet: Use hrtimer to do the retire operation", v6.18) turned it into "fire immediately and spin". Still present in 7.2.6 and in current mainline as of this writing. (Unrelated to CVE-2019-20812 - that one was the ethtool failure path returning 0, which now returns DEFAULT_PRB_RETIRE_TOV.) Reproducer, needs CAP_NET_RAW (a --network host container has it by default) and a 1 Gbps NIC (QEMU e1000 works): int fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL)); bind(fd, ...); int v = TPACKET_V3; setsockopt(fd, SOL_PACKET, PACKET_VERSION, &v, sizeof(v)); struct tpacket_req3 req = { .tp_block_size = 0x10000000, .tp_block_nr = 1, .tp_frame_size = 2048, .tp_frame_nr = 0x10000000 / 2048, .tp_retire_blk_tov = 0, }; setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &req, sizeof(req)); Compute in 64 bits instead. The operands are already bounded by the existing validation, so nothing else changes. If you'd prefer a different fix, just say so and I'll respin. Fixes: f7460d2989fa ("net: af_packet: Use hrtimer to do the retire operation") Cc: stable@vger.kernel.org Signed-off-by: Dairui Zhang --- First patch to netdev, and compile-tested only - I don't have a 1 Gbps setup to trigger it live. If I've misread the code, got the Fixes: tag wrong, or picked the wrong fix, please say so and I'll respin. net/packet/af_packet.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c index b22cda3..24cf2d2 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -617,7 +617,7 @@ static int prb_calc_retire_blk_tmo(struct packet_sock *po, return DEFAULT_PRB_RETIRE_TOV; div = ecmd.base.speed / 1000; - mbits = (blk_size_in_bytes * 8) / (1024 * 1024); + mbits = (u64)blk_size_in_bytes * 8 / (1024 * 1024); if (div) mbits /= div;