[PATCH bpf v7 06/10] bpf: Restrict CO-RE poisoning to relocatable instructions

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

CO-RE relocation records can name any instruction offset. When a
relocation cannot be resolved, bpf_core_patch_insn() currently poisons its
target before checking whether that instruction is a valid relocation
target. Malformed metadata can therefore replace jumps, calls, exits,
register-source arithmetic, or non-immediate loads instead of failing at
the relocation step.

Handle poisoning only after the instruction has passed the same class and
operand-form checks used for a resolved relocation. Route invalid forms
through the existing diagnostic and return a hard error. Keep poisoning
supported instructions, including both halves of a plain ldimm64, so an
unresolved relocation in dead code remains valid.

Extend bpf_core_poison_insn() to poison both halves of ldimm64, and return
its status directly from each validated instruction case. This avoids
routing the success path through a common label and leaves the helper free
to report errors.

The shared relocation code applies this restriction to both libbpf and
in-kernel CO-RE.

Fixes: d7a252708dbc ("libbpf: Improve handling of failed CO-RE relocations")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/relo_core.c | 58 +++++++++++++++++++++------------------
 1 file changed, 31 insertions(+), 27 deletions(-)

diff --git a/tools/lib/bpf/relo_core.c b/tools/lib/bpf/relo_core.c
index 8ad2715721cf..2672623a4198 100644
--- a/tools/lib/bpf/relo_core.c
+++ b/tools/lib/bpf/relo_core.c
@@ -980,23 +980,30 @@ static int bpf_core_calc_relo(const char *prog_name,
 }
 
 /*
- * Turn instruction for which CO_RE relocation failed into invalid one with
+ * Turn instruction for which CO-RE relocation failed into invalid one with
  * distinct signature.
  */
-static void bpf_core_poison_insn(const char *prog_name, int relo_idx,
-				 int insn_idx, struct bpf_insn *insn)
+static int bpf_core_poison_insn(const char *prog_name, int relo_idx,
+				struct bpf_insn *insn, int insn_idx)
 {
-	pr_debug("prog '%s': relo #%d: substituting insn #%d w/ invalid insn\n",
-		 prog_name, relo_idx, insn_idx);
-	insn->code = BPF_JMP | BPF_CALL;
-	insn->dst_reg = 0;
-	insn->src_reg = 0;
-	insn->off = 0;
-	/* if this instruction is reachable (not a dead code),
-	 * verifier will complain with the following message:
-	 * invalid func unknown#195896080
-	 */
-	insn->imm = 195896080; /* => 0xbad2310 => "bad relo" */
+	int insn_cnt = is_ldimm64_insn(insn) ? 2 : 1;
+	int i;
+
+	for (i = 0; i < insn_cnt; i++) {
+		pr_debug("prog '%s': relo #%d: substituting insn #%d w/ invalid insn\n",
+			 prog_name, relo_idx, insn_idx + i);
+		insn[i].code = BPF_JMP | BPF_CALL;
+		insn[i].dst_reg = 0;
+		insn[i].src_reg = 0;
+		insn[i].off = 0;
+		/*
+		 * If this instruction is reachable (not dead code), the verifier
+		 * will complain with "invalid func unknown#195896080".
+		 */
+		insn[i].imm = 195896080; /* => 0xbad2310 => "bad relo" */
+	}
+
+	return 0;
 }
 
 static int insn_bpf_size_to_bytes(struct bpf_insn *insn)
@@ -1047,17 +1054,6 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
 
 	class = BPF_CLASS(insn->code);
 
-	if (res->poison) {
-poison:
-		/* poison second part of ldimm64 to avoid confusing error from
-		 * verifier about "unknown opcode 00"
-		 */
-		if (is_ldimm64_insn(insn))
-			bpf_core_poison_insn(prog_name, relo_idx, insn_idx + 1, insn + 1);
-		bpf_core_poison_insn(prog_name, relo_idx, insn_idx, insn);
-		return 0;
-	}
-
 	orig_val = res->orig_val;
 	new_val = res->new_val;
 
@@ -1065,7 +1061,9 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
 	case BPF_ALU:
 	case BPF_ALU64:
 		if (BPF_SRC(insn->code) != BPF_K)
-			return -EINVAL;
+			goto bad_insn;
+		if (res->poison)
+			return bpf_core_poison_insn(prog_name, relo_idx, insn, insn_idx);
 		if (res->validate && insn->imm != orig_val) {
 			pr_warn("prog '%s': relo #%d: unexpected insn #%d (ALU/ALU64) value: got %d, exp %llu -> %llu\n",
 				prog_name, relo_idx,
@@ -1082,6 +1080,8 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
 	case BPF_LDX:
 	case BPF_ST:
 	case BPF_STX:
+		if (res->poison)
+			return bpf_core_poison_insn(prog_name, relo_idx, insn, insn_idx);
 		if (res->validate && insn->off != orig_val) {
 			pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDX/ST/STX) value: got %d, exp %llu -> %llu\n",
 				prog_name, relo_idx, insn_idx, insn->off, (unsigned long long)orig_val,
@@ -1097,7 +1097,7 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
 			pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) accesses field incorrectly. "
 				"Make sure you are accessing pointers, unsigned integers, or fields of matching type and size.\n",
 				prog_name, relo_idx, insn_idx);
-			goto poison;
+			return bpf_core_poison_insn(prog_name, relo_idx, insn, insn_idx);
 		}
 
 		orig_val = insn->off;
@@ -1140,6 +1140,9 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
 			return -EINVAL;
 		}
 
+		if (res->poison)
+			return bpf_core_poison_insn(prog_name, relo_idx, insn, insn_idx);
+
 		imm = (__u32)insn[0].imm | ((__u64)insn[1].imm << 32);
 		if (res->validate && imm != orig_val) {
 			pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %llu -> %llu\n",
@@ -1157,6 +1160,7 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
 		break;
 	}
 	default:
+bad_insn:
 		pr_warn("prog '%s': relo #%d: trying to relocate unrecognized insn #%d, code:0x%x, src:0x%x, dst:0x%x, off:0x%x, imm:0x%x\n",
 			prog_name, relo_idx, insn_idx, insn->code,
 			(unsigned)insn->src_reg, (unsigned)insn->dst_reg, (unsigned)insn->off, (unsigned)insn->imm);
-- 
2.53.0