Messages in this thread Patch in this message | | | Subject | Re: [PATCH bpf-next v2 1/2] bpf: Fix a umin > umax reg bound error | From | Daniel Borkmann <> | Date | Mon, 20 Mar 2023 17:42:24 +0100 |
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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));
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