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Jamal Hadi Salim says: ==================== net: cap tx_queue_len at S16_MAX to prevent oversized ring allocations An unprivileged user (via unshare -Urn) can set a huge tx_queue_len and exhaust global memory through ring allocations sized from it (pfifo_fast skb_arrays, tun/tap ptr_rings). The reproducer from vega@nebusec.ai set the following params for illustration: txqlen of 500000 -> ~32 GiB/ring attempts, 1.6 GB tun, ~960 MB tap. Gets worse when you consider qdiscs like mq. What we fix: every path an unprivileged user can use to install an oversized tx_queue_len is rejected with -ERANGE before any ring is allocated; per-ring memory is bounded at 256 KiB. This is for you sashikos: What we deliberately _do not fix_ bound the NUMBER of rings. With the cap in place the worst case moves from "one knob" to the aggregate of ring x queues x devices, example: ip link add v0 numtxqueues 4096 txqueuelen 32767 type veth tc qdisc add dev v0 root mq -> 4096 * 3 * 32767 * 8 = ~3.0 GiB (one command) 50 tun devices x 256 queues x 32767 x 8 = ~3.1 GiB Unfortunately tx_queue_len is a bit ambigious in meaning: In some cases it means a ring size (which is pre-allocated, ex: tun, tap, and pfifo_fast); a cap of 4096 seems reasonable here. but in other cases it is used to indicate a queue limit ex: the qdisc consumers that allocate nothing (pfifo/bfifo/gred/plug/sfb, htb direct_qlen, qfq, teql). 32767 is a legitimate high-BDP queue length, so we are going to keep that value. Getting back to you sashikos, after this is merged and shows up in net-next we will send followup patches as follows: this series is not misread as "closes the OOM class"): a) Per-site ring limits at six identified locations - pfifo_fast init/resize, - tun attach/resize, - tap minor/resize) if you can spot more in your review we will take care of those as well. b) memcg accounting (GFP_KERNEL_ACCOUNT) for those ring allocations: contains a memcg-limited container's ring memory. Not GFP_KERNEL_ACCOUNT has no effect on the unshare attacker but will protect against containers (memory.max in its cgroup) ==================== Link: https://patch.msgid.link/QDISC-2899.v2.20260901233641@mojatatu.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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