bpf: enhance verifier to understand stack pointer arithmetic
Subsystems (6)
PATCH 1/1 bpf: enhance verifier to understand stack pointer arithmetic
From: Yonghong Song <[email protected]>
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.
---
technically it's 'net' material, but it's too late for 'net', hence 'net-next' tag.
No 'Fixes' tag, since it's only seen with newer llvm.
--- kernel/bpf/verifier.c | 11 +++++++++++ tools/testing/selftests/bpf/test_verifier.c | 18 ++++++++++++------ 2 files changed, 23 insertions(+), 6 deletions(-)
kernel/bpf/verifier.c +11 -0
@@ -1924,6 +1924,17 @@ 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 ))) {
tools/testing/selftests/bpf/test_verifier.c +12 -6
@@ -1814,16 +1814,22 @@ 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, }, {
Signature · 1 line
2.9.3
From: Alexei Starovoitov <[email protected]>
Date: Sat, 29 Apr 2017 22:52:42 -0700
Quoted 21 lines
From: Yonghong Song <[email protected]>
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 validstack_ptrwith new offset.
Quoted 8 lines
Signed-off-by Yonghong Song <[email protected]>Acked-by Martin KaFai Lau <[email protected]>Acked-by Daniel Borkmann <[email protected]>Signed-off-by Alexei Starovoitov <[email protected]>---
technically it's 'net' material, but it's too late for 'net', hence 'net-next' tag.
No 'Fixes' tag, since it's only seen with newer llvm.
Applied to net-next, but I'll queue this up to -stable.