bpf: enhance verifier to understand stack pointer arithmetic

[ Upstream commit 332270fdc8b6fba07d059a9ad44df9e1a2ad4529 ]

llvm 4.0 and above generates the code like below:
....
440: (b7) r1 = 15
441: (05) goto pc+73
515: (79) r6 = *(u64 *)(r10 -152)
516: (bf) r7 = r10
517: (07) r7 += -112
518: (bf) r2 = r7
519: (0f) r2 += r1
520: (71) r1 = *(u8 *)(r8 +0)
521: (73) *(u8 *)(r2 +45) = r1
....
and the verifier complains "R2 invalid mem access 'inv'" for insn #521.
This is because verifier marks register r2 as unknown value after #519
where r2 is a stack pointer and r1 holds a constant value.

Teach verifier to recognize "stack_ptr + imm" and
"stack_ptr + reg with const val" as valid stack_ptr with new offset.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 23 insertions, 6 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7c9f94c53441..64fcab1d8cd9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1749,6 +1749,17 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
                         return 0;
                 } else if (opcode == BPF_ADD &&
                            BPF_CLASS(insn->code) == BPF_ALU64 &&
+                           dst_reg->type == PTR_TO_STACK &&
+                           ((BPF_SRC(insn->code) == BPF_X &&
+                             regs[insn->src_reg].type == CONST_IMM) ||
+                            BPF_SRC(insn->code) == BPF_K)) {
+                        if (BPF_SRC(insn->code) == BPF_X)
+                                dst_reg->imm += regs[insn->src_reg].imm;
+                        else
+                                dst_reg->imm += insn->imm;
+                        return 0;
+                } else if (opcode == BPF_ADD &&
+                           BPF_CLASS(insn->code) == BPF_ALU64 &&
                            (dst_reg->type == PTR_TO_PACKET ||
                             (BPF_SRC(insn->code) == BPF_X &&
                              regs[insn->src_reg].type == PTR_TO_PACKET))) {
diff --git a/samples/bpf/test_verifier.c b/samples/bpf/test_verifier.c
index 369ffaad3799..dc7dec9e64ba 100644
--- a/samples/bpf/test_verifier.c
+++ b/samples/bpf/test_verifier.c
@@ -1218,16 +1218,22 @@ static struct bpf_test tests[] = {
                 .result = ACCEPT,
         },
         {
-                "unpriv: obfuscate stack pointer",
+                "stack pointer arithmetic",
                 .insns = {
-                        BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
-                        BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
-                        BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+                        BPF_MOV64_IMM(BPF_REG_1, 4),
+                        BPF_JMP_IMM(BPF_JA, 0, 0, 0),
+                        BPF_MOV64_REG(BPF_REG_7, BPF_REG_10),
+                        BPF_ALU64_IMM(BPF_ADD, BPF_REG_7, -10),
+                        BPF_ALU64_IMM(BPF_ADD, BPF_REG_7, -10),
+                        BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
+                        BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1),
+                        BPF_ST_MEM(0, BPF_REG_2, 4, 0),
+                        BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
+                        BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, 8),
+                        BPF_ST_MEM(0, BPF_REG_2, 4, 0),
                         BPF_MOV64_IMM(BPF_REG_0, 0),
                         BPF_EXIT_INSN(),
                 },
-                .errstr_unpriv = "R2 pointer arithmetic",
-                .result_unpriv = REJECT,
                 .result = ACCEPT,
         },
         {