Commit 332270fd authored by Yonghong Song's avatar Yonghong Song Committed by David S. Miller
Browse files

bpf: enhance verifier to understand stack pointer arithmetic 



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: default avatarYonghong Song <yhs@fb.com>
Acked-by: default avatarMartin KaFai Lau <kafai@fb.com>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 2faf2657

kernel/bpf/verifier.c

+11 −0
Original line number Diff line number Diff line
@@ -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->imm = insn->imm;

			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 ||

tools/testing/selftests/bpf/test_verifier.c

+12 −6
Original line number Diff line number Diff line
@@ -1932,16 +1932,22 @@ 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,

	},

	{

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