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bpf: enhance verifier to understand stack pointer arithmetic

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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>
This commit is contained in:
Yonghong Song 2017-04-29 22:52:42 -07:00 • committed by David S. Miller
parent 2faf265753
commit 332270fdc8
2 changed files with 23 additions and 6 deletions
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11
kernel/bpf/verifier.c
View file

@ -1922,6 +1922,17 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
dst_reg->type = PTR_TO_STACK; dst_reg->type = PTR_TO_STACK;
dst_reg->imm = insn->imm; dst_reg->imm = insn->imm;
return 0; 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 && } else if (opcode == BPF_ADD &&
BPF_CLASS(insn->code) == BPF_ALU64 && BPF_CLASS(insn->code) == BPF_ALU64 &&
(dst_reg->type == PTR_TO_PACKET || (dst_reg->type == PTR_TO_PACKET ||

18
tools/testing/selftests/bpf/test_verifier.c
View file

@ -1932,16 +1932,22 @@ static struct bpf_test tests[] = {
.result = ACCEPT, .result = ACCEPT,
}, },
{ {
"unpriv: obfuscate stack pointer", "stack pointer arithmetic",
.insns = { .insns = {
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), BPF_MOV64_IMM(BPF_REG_1, 4),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), BPF_JMP_IMM(BPF_JA, 0, 0, 0),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), 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_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(), BPF_EXIT_INSN(),
}, },
.errstr_unpriv = "R2 pointer arithmetic",
.result_unpriv = REJECT,
.result = ACCEPT, .result = ACCEPT,
}, },
{ {