[RFC PATCH] bpf: handle 32-bit zext during constant blinding

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Since BPF constant blinding is performed after the verifier pass, there
are certain ALU32 instructions inserted which don't have a corresponding
zext instruction inserted after. This is causing a kernel oops on
powerpc and can be reproduced by running 'test_cgroup_storage' with
bpf_jit_harden=2.

Fix this by emitting BPF_ZEXT during constant blinding if
prog->aux->verifier_zext is set.

Fixes: a4b1d3c1ddf6cb ("bpf: verifier: insert zero extension according to analysis result")
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
This approach (the location where zext is being introduced below, in 
particular) works for powerpc, but I am not entirely sure if this is 
sufficient for other architectures as well. This is broken on v5.3-rc4.

- Naveen


 kernel/bpf/core.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 8191a7db2777..d84146e6fd9e 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -890,7 +890,8 @@ int bpf_jit_get_func_addr(const struct bpf_prog *prog,
 
 static int bpf_jit_blind_insn(const struct bpf_insn *from,
 			      const struct bpf_insn *aux,
-			      struct bpf_insn *to_buff)
+			      struct bpf_insn *to_buff,
+			      bool emit_zext)
 {
 	struct bpf_insn *to = to_buff;
 	u32 imm_rnd = get_random_int();
@@ -939,6 +940,8 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
 		*to++ = BPF_ALU32_IMM(BPF_MOV, BPF_REG_AX, imm_rnd ^ from->imm);
 		*to++ = BPF_ALU32_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
 		*to++ = BPF_ALU32_REG(from->code, from->dst_reg, BPF_REG_AX);
+		if (emit_zext)
+			*to++ = BPF_ZEXT_REG(from->dst_reg);
 		break;
 
 	case BPF_ALU64 | BPF_ADD | BPF_K:
@@ -992,6 +995,10 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
 			off -= 2;
 		*to++ = BPF_ALU32_IMM(BPF_MOV, BPF_REG_AX, imm_rnd ^ from->imm);
 		*to++ = BPF_ALU32_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
+		if (emit_zext) {
+			*to++ = BPF_ZEXT_REG(BPF_REG_AX);
+			off--;
+		}
 		*to++ = BPF_JMP32_REG(from->code, from->dst_reg, BPF_REG_AX,
 				      off);
 		break;
@@ -1005,6 +1012,8 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
 	case 0: /* Part 2 of BPF_LD | BPF_IMM | BPF_DW. */
 		*to++ = BPF_ALU32_IMM(BPF_MOV, BPF_REG_AX, imm_rnd ^ aux[0].imm);
 		*to++ = BPF_ALU32_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
+		if (emit_zext)
+			*to++ = BPF_ZEXT_REG(BPF_REG_AX);
 		*to++ = BPF_ALU64_REG(BPF_OR,  aux[0].dst_reg, BPF_REG_AX);
 		break;
 
@@ -1088,7 +1097,8 @@ struct bpf_prog *bpf_jit_blind_constants(struct bpf_prog *prog)
 		    insn[1].code == 0)
 			memcpy(aux, insn, sizeof(aux));
 
-		rewritten = bpf_jit_blind_insn(insn, aux, insn_buff);
+		rewritten = bpf_jit_blind_insn(insn, aux, insn_buff,
+						clone->aux->verifier_zext);
 		if (!rewritten)
 			continue;
 
-- 
2.22.0