Messages in this thread | | | Date | Tue, 21 Mar 2023 16:28:27 +0800 | Subject | Re: [PATCH bpf-next v2 1/2] bpf: Fix a umin > umax reg bound error | From | Xu Kuohai <> |
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On 3/21/2023 12:42 AM, Daniel Borkmann wrote: > On 3/17/23 11:24 PM, Daniel Borkmann wrote: >> On 3/14/23 9:34 PM, Xu Kuohai wrote: >>> From: Xu Kuohai <xukuohai@huawei.com> >>> >>> After commit 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking"), >>> the following bpf prog is rejected: >>> >>> 0: (61) r2 = *(u32 *)(r1 +0) ; R2_w=pkt(off=0,r=0,imm=0) >>> 1: (61) r3 = *(u32 *)(r1 +4) ; R3_w=pkt_end(off=0,imm=0) >>> 2: (bf) r1 = r2 >>> 3: (07) r1 += 1 >>> 4: (2d) if r1 > r3 goto pc+8 >>> 5: (71) r1 = *(u8 *)(r2 +0) ; R1_w=scalar(umax=255,var_off=(0x0; 0xff)) >>> 6: (18) r0 = 0x7fffffffffffff10 >>> 8: (0f) r1 += r0 ; R1_w=scalar(umin=0x7fffffffffffff10,umax=0x800000000000000f) >>> 9: (18) r0 = 0x8000000000000000 >>> 11: (07) r0 += 1 >>> 12: (ad) if r0 < r1 goto pc-2 >>> 13: (b7) r0 = 0 >>> 14: (95) exit >>> >>> And the verifier log says: >>> >>> [...] >>> >>> from 12 to 11: R0_w=-9223372036854775794 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) >>> 11: (07) r0 += 1 ; R0_w=-9223372036854775793 >>> 12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) >>> 13: safe >>> >>> from 12 to 11: R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) >>> 11: (07) r0 += 1 ; R0_w=-9223372036854775792 >>> 12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775792 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff)) >>> 13: safe >>> >>> [...] >>> >>> What can be seen here is that r1->umin grows blindly and becomes bigger >>> than r1->umax. The reason is because the loop does not terminate, when >>> r0 increases to r1->umax_value, the following code in reg_set_min_max() >>> sets r1->umin_value to r1->umax_value + 1 blindly: >>> >>> case BPF_JGT: >>> { >>> if (is_jmp32) { >>> [...] >>> } else { >>> u64 false_umax = opcode == BPF_JGT ? val : val - 1; >>> u64 true_umin = opcode == BPF_JGT ? val + 1 : val; >>> >>> false_reg->umax_value = min(false_reg->umax_value, false_umax); >>> true_reg->umin_value = max(true_reg->umin_value, true_umin); >>> } >>> break; >>> } >>> >>> Why the loop does not terminate is because tnum_is_const(src_reg->var_off) >>> always returns false, causing is_branch_taken() to be skipped: >>> >>> if (src_reg->type == SCALAR_VALUE && >>> !is_jmp32 && tnum_is_const(src_reg->var_off)) { >>> pred = is_branch_taken(dst_reg, // could not reach here >>> src_reg->var_off.value, >>> opcode, >>> is_jmp32); >>> } >>> >>> Why tnum_is_const(src_reg->var_off) always returns false is because >>> r1->umin_value starts increasing from 0x7fffffffffffff10, always bigger >>> than U32_MAX, causing the __reg_combine_64_into_32() to mark the lower >>> 32 bits unbounded, i.e. not a constant. >>> >>> To fix it: >>> 1. avoid increasing reg lower bound to a value bigger than the upper bound, >>> or decreasing reg upper bound to a value smaller than the lower bound. >>> 2. set 32-bit min/max values to the lower 32 bits of the 64-bit min/max values >>> when the 64-bit min/max values are equal. >> >> Should both these be separate patches, meaning are both of them strictly >> required as one logical entity or not? From your description it's not really >> clear wrt reg_{inc,dec}_{u32,u64}_{min,max} and if this is mainly defensive >> or required. > > Fyi, I'm working on the below draft patch which passes all of test_verifier and > your test cases as well from patch 2. Will cook a proper patch once I'm through > with further analysis: > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index d517d13878cf..8bef2ed89e87 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -1823,7 +1823,7 @@ static void __reg_bound_offset(struct bpf_reg_state *reg) > struct tnum var64_off = tnum_intersect(reg->var_off, > tnum_range(reg->umin_value, > reg->umax_value)); > - struct tnum var32_off = tnum_intersect(tnum_subreg(reg->var_off), > + struct tnum var32_off = tnum_intersect(tnum_subreg(var64_off), > tnum_range(reg->u32_min_value, > reg->u32_max_value)); > .
[forget to reply to the list, resend]
Thanks for the patch, it works for me. But as replied in the other mail, it seems more reasonable to converge var32_off to constant by converging [u32_min_value, u32_max_value] to constant.
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