From: Jiong Wang jiong.wang@netronome.com

[ Upstream commit e434b8cdf788568ba65a0a0fd9f3cb41f3ca1803 ]

Currently, the destination register is marked as unknown for 32-bit sub-register move (BPF_MOV | BPF_ALU) whenever the source register type is SCALAR_VALUE.

This is too conservative that some valid cases will be rejected. Especially, this may turn a constant scalar value into unknown value that could break some assumptions of verifier.

For example, test_l4lb_noinline.c has the following C code:

struct real_definition *dst

1: if (!get_packet_dst(&dst, &pckt, vip_info, is_ipv6)) 2: return TC_ACT_SHOT; 3: 4: if (dst->flags & F_IPV6) {

get_packet_dst is responsible for initializing "dst" into valid pointer and return true (1), otherwise return false (0). The compiled instruction sequence using alu32 will be:

412: (54) (u32) r7 &= (u32) 1 413: (bc) (u32) r0 = (u32) r7 414: (95) exit

insn 413, a BPF_MOV | BPF_ALU, however will turn r0 into unknown value even r7 contains SCALAR_VALUE 1.

This causes trouble when verifier is walking the code path that hasn't initialized "dst" inside get_packet_dst, for which case 0 is returned and we would then expect verifier concluding line 1 in the above C code pass the "if" check, therefore would skip fall through path starting at line 4. Now, because r0 returned from callee has became unknown value, so verifier won't skip analyzing path starting at line 4 and "dst->flags" requires dereferencing the pointer "dst" which actually hasn't be initialized for this path.

This patch relaxed the code marking sub-register move destination. For a SCALAR_VALUE, it is safe to just copy the value from source then truncate it into 32-bit.

A unit test also included to demonstrate this issue. This test will fail before this patch.

This relaxation could let verifier skipping more paths for conditional comparison against immediate. It also let verifier recording a more accurate/strict value for one register at one state, if this state end up with going through exit without rejection and it is used for state comparison later, then it is possible an inaccurate/permissive value is better. So the real impact on verifier processed insn number is complex. But in all, without this fix, valid program could be rejected.

From real benchmarking on kernel selftests and Cilium bpf tests, there is

no impact on processed instruction number when tests ares compiled with default compilation options. There is slightly improvements when they are compiled with -mattr=+alu32 after this patch.

Also, test_xdp_noinline/-mattr=+alu32 now passed verification. It is rejected before this fix.

Insn processed before/after this patch:

default -mattr=+alu32

Kernel selftest

=== test_xdp.o 371/371 369/369 test_l4lb.o 6345/6345 5623/5623 test_xdp_noinline.o 2971/2971 rejected/2727 test_tcp_estates.o 429/429 430/430

Cilium bpf === bpf_lb-DLB_L3.o: 2085/2085 1685/1687 bpf_lb-DLB_L4.o: 2287/2287 1986/1982 bpf_lb-DUNKNOWN.o: 690/690 622/622 bpf_lxc.o: 95033/95033 N/A bpf_netdev.o: 7245/7245 N/A bpf_overlay.o: 2898/2898 3085/2947

NOTE: - bpf_lxc.o and bpf_netdev.o compiled by -mattr=+alu32 are rejected by verifier due to another issue inside verifier on supporting alu32 binary. - Each cilium bpf program could generate several processed insn number, above number is sum of them.

v1->v2: - Restrict the change on SCALAR_VALUE. - Update benchmark numbers on Cilium bpf tests.

Signed-off-by: Jiong Wang jiong.wang@netronome.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/verifier.c | 16 ++++++++++++---- tools/testing/selftests/bpf/test_verifier.c | 13 +++++++++++++ 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 89cea3ed535d..341806668f03 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -3285,12 +3285,15 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn) return err;

if (BPF_SRC(insn->code) == BPF_X) { + struct bpf_reg_state *src_reg = regs + insn->src_reg; + struct bpf_reg_state *dst_reg = regs + insn->dst_reg; + if (BPF_CLASS(insn->code) == BPF_ALU64) { /* case: R1 = R2 * copy register state to dest reg */ - regs[insn->dst_reg] = regs[insn->src_reg]; - regs[insn->dst_reg].live |= REG_LIVE_WRITTEN; + *dst_reg = *src_reg; + dst_reg->live |= REG_LIVE_WRITTEN; } else { /* R1 = (u32) R2 */ if (is_pointer_value(env, insn->src_reg)) { @@ -3298,9 +3301,14 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn) "R%d partial copy of pointer\n", insn->src_reg); return -EACCES; + } else if (src_reg->type == SCALAR_VALUE) { + *dst_reg = *src_reg; + dst_reg->live |= REG_LIVE_WRITTEN; + } else { + mark_reg_unknown(env, regs, + insn->dst_reg); } - mark_reg_unknown(env, regs, insn->dst_reg); - coerce_reg_to_size(&regs[insn->dst_reg], 4); + coerce_reg_to_size(dst_reg, 4); } } else { /* case: R = imm diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c index e436b67f2426..9db5a7378f40 100644 --- a/tools/testing/selftests/bpf/test_verifier.c +++ b/tools/testing/selftests/bpf/test_verifier.c @@ -2748,6 +2748,19 @@ static struct bpf_test tests[] = { .result_unpriv = REJECT, .result = ACCEPT, }, + { + "alu32: mov u32 const", + .insns = { + BPF_MOV32_IMM(BPF_REG_7, 0), + BPF_ALU32_IMM(BPF_AND, BPF_REG_7, 1), + BPF_MOV32_REG(BPF_REG_0, BPF_REG_7), + BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1), + BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_7, 0), + BPF_EXIT_INSN(), + }, + .result = ACCEPT, + .retval = 0, + }, { "unpriv: partial copy of pointer", .insns = {