bpf: net_sched: Disable attaching bpf qdisc to non root

Do not allow users to attach bpf qdiscs to classful qdiscs. This is to
prevent accidentally breaking existings classful qdiscs if they rely on
some data in the child qdisc. This restriction can potentially be lifted
in the future. Note that, we still allow bpf qdisc to be attached to mq.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20250409214606.2000194-7-ameryhung@gmail.com

diff --git a/net/sched/bpf_qdisc.c b/net/sched/bpf_qdisc.c
index dcbd1c9276345468d0cffbe40af6630a6a5c902d..9f32b305636fcbc06c75376d4e2a627ec3b6b8af 100644 (file)
--- a/net/sched/bpf_qdisc.c
+++ b/net/sched/bpf_qdisc.c
@@ -142,13 +142,19 @@ static int bpf_qdisc_gen_prologue(struct bpf_insn *insn_buf, bool direct_write,
                return 0;
 
        /* r6 = r1; // r6 will be "u64 *ctx". r1 is "u64 *ctx".
+        * r2 = r1[16]; // r2 will be "struct netlink_ext_ack *extack"
         * r1 = r1[0]; // r1 will be "struct Qdisc *sch"
-        * r0 = bpf_qdisc_init_prologue(r1);
+        * r0 = bpf_qdisc_init_prologue(r1, r2);
+        * if r0 == 0 goto pc+1;
+        * BPF_EXIT;
         * r1 = r6; // r1 will be "u64 *ctx".
         */
        *insn++ = BPF_MOV64_REG(BPF_REG_6, BPF_REG_1);
+       *insn++ = BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1, 16);
        *insn++ = BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, 0);
        *insn++ = BPF_CALL_KFUNC(0, bpf_qdisc_init_prologue_ids[0]);
+       *insn++ = BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1);
+       *insn++ = BPF_EXIT_INSN();
        *insn++ = BPF_MOV64_REG(BPF_REG_1, BPF_REG_6);
        *insn++ = prog->insnsi[0];
 
@@ -221,11 +227,26 @@ __bpf_kfunc void bpf_qdisc_watchdog_schedule(struct Qdisc *sch, u64 expire, u64
 }
 
 /* bpf_qdisc_init_prologue - Hidden kfunc called in prologue of .init. */
-__bpf_kfunc void bpf_qdisc_init_prologue(struct Qdisc *sch)
+__bpf_kfunc int bpf_qdisc_init_prologue(struct Qdisc *sch,
+                                       struct netlink_ext_ack *extack)
 {
        struct bpf_sched_data *q = qdisc_priv(sch);
+       struct net_device *dev = qdisc_dev(sch);
+       struct Qdisc *p;
+
+       if (sch->parent != TC_H_ROOT) {
+               p = qdisc_lookup(dev, TC_H_MAJ(sch->parent));
+               if (!p)
+                       return -ENOENT;
+
+               if (!(p->flags & TCQ_F_MQROOT)) {
+                       NL_SET_ERR_MSG(extack, "BPF qdisc only supported on root or mq");
+                       return -EINVAL;
+               }
+       }
 
        qdisc_watchdog_init(&q->watchdog, sch);
+       return 0;
 }
 
 /* bpf_qdisc_reset_destroy_epilogue - Hidden kfunc called in epilogue of .reset