[PATCH bpf-next] Detect jumping to reserved code during check_cfg()

[Hao Sun]

Currently, we don't check if the branch-taken of a jump is reserved code of
ld_imm64. Instead, such a issue is captured in check_ld_imm(). The verifier
gives the following log in such case:

func#0 @0
0: R1=ctx(off=0,imm=0) R10=fp0
0: (18) r4 = 0xffff888103436000       ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
2: (18) r1 = 0x1d                     ; R1_w=29
4: (55) if r4 != 0x0 goto pc+4        ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
5: (1c) w1 -= w1                      ; R1_w=0
6: (18) r5 = 0x32                     ; R5_w=50
8: (56) if w5 != 0xfffffff4 goto pc-2
mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx -1
mark_precise: frame0: regs=r5 stack= before 6: (18) r5 = 0x32
7: R5_w=50
7: BUG_ld_00
invalid BPF_LD_IMM insn

Here the verifier rejects the program because it thinks insn at 7 is an
invalid BPF_LD_IMM, but such a error log is not accurate since the issue
is jumping to reserved code not because the program contains invalid insn.
Therefore, make the verifier check the jump target during check_cfg(). For
the same program, the verifier reports the following log:

func#0 @0
jump to reserved code from insn 8 to 7

---


Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index eed7350e15f4..725ac0b464cf 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14980,6 +14980,7 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
 {
 	int *insn_stack = env->cfg.insn_stack;
 	int *insn_state = env->cfg.insn_state;
+	struct bpf_insn *insns = env->prog->insnsi;
 
 	if (e == FALLTHROUGH && insn_state[t] >= (DISCOVERED | FALLTHROUGH))
 		return DONE_EXPLORING;
@@ -14993,6 +14994,12 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
 		return -EINVAL;
 	}
 
+	if (e == BRANCH && insns[w].code == 0) {
+		verbose_linfo(env, t, "%d", t);
+		verbose(env, "jump to reserved code from insn %d to %d\n", t, w);
+		return -EINVAL;
+	}
+
 	if (e == BRANCH) {
 		/* mark branch target for state pruning */
 		mark_prune_point(env, w);

---
base-commit: 3157b7ce14bbf468b0ca8613322a05c37b5ae25d
change-id: 20231009-jmp-into-reserved-fields-fc1a98a8e7dc

Best regards,
-- 
Hao Sun <sunhao.th@xxxxxxxxx>