Add kernelCTF CVE-2023-31436

pocs/linux/kernelctf/CVE-2023-31436_mitigation/docs/novel-techniques.md

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+As far as I am aware kernel heap data only attacks using kernel/user shared memory have
+gained little to no attention yet.
+
+The `struct xdp_mem` with the `AF_XDP` socket ([docs](https://www.kernel.org/doc/html/latest/networking/af_xdp.html))
+used in this exploit seems to serve as a powerful primitive:
+Advantages:
+- Read / Write of "kernel" memory without restrictions (even fault shenanigans seem like a good idea,
+    though I did not explicitly look into this)
+- No heap pointers required
+- No size restrictions even though acting as a `kmalloc-128` object
+- Very useful as a "fake list member"
+
+Disadvantages:
+- One level of pointer indirection
+- Initial pointer restricted to `kmalloc-128`
+- Requires `CAP_NET_RAW`
+
+Depending on the primitives available unaligned pointer corruption may come in handy
+when dealing with objects where the mapped member does not align with the desired pointer.

pocs/linux/kernelctf/CVE-2023-31436_mitigation/docs/vulnerability.md

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+Vulnerability Details
+=====================
+
+CVE-2023-31436: qfq_change_class in net/sched/sch_qfq.c in the Linux kernel before 6.2.13 allows an out-of-bounds write because lmax can exceed QFQ_MIN_LMAX.
+
+This vulnerability affects the packet scheduler subsystem, specifically QFQ+.
+
+An attacker can utilize this vulnerability to cause a slab-out-of-bounds read/write in the `(dyn-)kmalloc-8192` cache.
+
+## Requirements
+
+A user needs to be able to modify qdiscs, thus requiring `CAP_NET_ADMIN`.
+Naturally this will be obtained through usernamespaces, thus one may require `CONFIG_USER_NS`.
+
+The specific qdisc in question is QFQ, which needs to be enabled `CONFIG_NET_SCH_QFQ`.
+
+## History
+
+The fixing commit is https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3037933448f60f9acb705997eae62013ecb81e0d.
+This is a fix for 3015f3d2a3cd ("pkt_sched: enable QFQ to support TSO/GSO"), which dates way back to 2011.
+Based on this I assume 2.6.x+ is affected.
+
+## Triggering the Vulnerability
+
+In order to trigger the vulnerability an attacker needs to modify qfq classes
+after modifying the MTU of the device to a large value (> 0x100000).
+This can be trivially achieved for the loopback device.
+
+When changing a class and ommitting the `TCA_QFQ_LMAX` option, the `lmax` value is chosen according to the MTU of the device, without any additional checks [1]:
+```c
+// qfq_change_class() in net/sched/sch_qfq.c
+
+// ..
+	if (tb[TCA_QFQ_LMAX]) {
+		lmax = nla_get_u32(tb[TCA_QFQ_LMAX]);
+		if (lmax < QFQ_MIN_LMAX || lmax > (1UL << QFQ_MTU_SHIFT)) {
+			pr_notice("qfq: invalid max length %u\n", lmax);
+			return -EINVAL;
+		}
+	} else
+		lmax = psched_mtu(qdisc_dev(sch)); // [1]
+
+// ..
+
+		qfq_init_agg(q, new_agg, lmax, weight); // [2]
+	}
+
+// ..
+
+	qfq_add_to_agg(q, new_agg, cl); // [3]
+```
+
+`qfq_init_agg` will then set `new_agg->lmax` accordingly.
+Eventually `qfq_add_to_agg()` [3] will initialize `new_agg->grp` when the call tree
+reaches `qfq_update_agg()`:
+
+```c
+// qfq_update_agg() in net/sched/sch_qfq.c
+	agg->budgetmax = new_num_classes * agg->lmax;
+	new_agg_weight = agg->class_weight * new_num_classes;
+	agg->inv_w = ONE_FP/new_agg_weight;
+
+	if (agg->grp == NULL) {
+		int i = qfq_calc_index(agg->inv_w, agg->budgetmax,
+				       q->min_slot_shift);
+		agg->grp = &q->groups[i]; // [4]
+	}
+```
+
+`qfq_calc_index()` performs some simple arithmetics to choose the final value,
+but will not do any additional bounds checks.
+Eventually this results in `agg->grp` pointing out-of-bounds [4] relative to the `q` object of type `struct qfq_sched` (in the `kmalloc-8192` cache).
+
+The group of the `qfq_aggregate` is used in several places, leading to OOB reads and writes.

pocs/linux/kernelctf/CVE-2023-31436_mitigation/exploit/mitigation-6.1/Makefile

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+all: exploit.c bin
+	$(CC) exploit.c -o bin/exploit -O3 -static
+
+exploit: exploit.c
+	$(CC) exploit.c -o exploit -O3 -static
+
+bin:
+	mkdir -p bin/
+
+run:
+	./bin/exploit