From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yonghong Song Subject: Re: [PATCH bpf-next v3 3/9] bpf/verifier: refine retval R0 state for bpf_get_stack helper Date: Sun, 22 Apr 2018 19:46:24 -0700 Message-ID: <35fb9a8c-06f1-e781-714c-7a7cf085e5a1@fb.com> References: <20180420221842.742330-1-yhs@fb.com> <20180420221842.742330-4-yhs@fb.com> <20180422235538.5tqayfahfeqanfou@ast-mbp> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Cc: , , , To: Alexei Starovoitov Return-path: Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:32944 "EHLO mx0b-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753870AbeDWCq5 (ORCPT ); Sun, 22 Apr 2018 22:46:57 -0400 In-Reply-To: <20180422235538.5tqayfahfeqanfou@ast-mbp> Content-Language: en-US Sender: netdev-owner@vger.kernel.org List-ID: On 4/22/18 4:55 PM, Alexei Starovoitov wrote: > On Fri, Apr 20, 2018 at 03:18:36PM -0700, Yonghong Song wrote: >> The special property of return values for helpers bpf_get_stack >> and bpf_probe_read_str are captured in verifier. >> Both helpers return a negative error code or >> a length, which is equal to or smaller than the buffer >> size argument. This additional information in the >> verifier can avoid the condition such as "retval > bufsize" >> in the bpf program. For example, for the code blow, >> usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK); >> if (usize < 0 || usize > max_len) >> return 0; >> The verifier may have the following errors: >> 52: (85) call bpf_get_stack#65 >> R0=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R1_w=ctx(id=0,off=0,imm=0) >> R2_w=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R3_w=inv800 R4_w=inv256 >> R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0) >> R9_w=inv800 R10=fp0,call_-1 >> 53: (bf) r8 = r0 >> 54: (bf) r1 = r8 >> 55: (67) r1 <<= 32 >> 56: (bf) r2 = r1 >> 57: (77) r2 >>= 32 >> 58: (25) if r2 > 0x31f goto pc+33 >> R0=inv(id=0) R1=inv(id=0,smax_value=9223372032559808512, >> umax_value=18446744069414584320, >> var_off=(0x0; 0xffffffff00000000)) >> R2=inv(id=0,umax_value=799,var_off=(0x0; 0x3ff)) >> R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0) >> R8=inv(id=0) R9=inv800 R10=fp0,call_-1 >> 59: (1f) r9 -= r8 >> 60: (c7) r1 s>>= 32 >> 61: (bf) r2 = r7 >> 62: (0f) r2 += r1 >> math between map_value pointer and register with unbounded >> min value is not allowed >> The failure is due to llvm compiler optimization where register "r2", >> which is a copy of "r1", is tested for condition while later on "r1" >> is used for map_ptr operation. The verifier is not able to track such >> inst sequence effectively. >> >> Without the "usize > max_len" condition, there is no llvm optimization >> and the below generated code passed verifier: >> 52: (85) call bpf_get_stack#65 >> R0=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R1_w=ctx(id=0,off=0,imm=0) >> R2_w=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R3_w=inv800 R4_w=inv256 >> R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0) >> R9_w=inv800 R10=fp0,call_-1 >> 53: (b7) r1 = 0 >> 54: (bf) r8 = r0 >> 55: (67) r8 <<= 32 >> 56: (c7) r8 s>>= 32 >> 57: (6d) if r1 s> r8 goto pc+24 >> R0=inv(id=0,umax_value=800) R1=inv0 R6=ctx(id=0,off=0,imm=0) >> R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0) >> R8=inv(id=0,umax_value=800,var_off=(0x0; 0x3ff)) R9=inv800 >> R10=fp0,call_-1 >> 58: (bf) r2 = r7 >> 59: (0f) r2 += r8 >> 60: (1f) r9 -= r8 >> 61: (bf) r1 = r6 >> >> Signed-off-by: Yonghong Song >> --- >> kernel/bpf/verifier.c | 27 +++++++++++++++++++++++++++ >> 1 file changed, 27 insertions(+) >> >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >> index aba9425..3c8bb92 100644 >> --- a/kernel/bpf/verifier.c >> +++ b/kernel/bpf/verifier.c >> @@ -164,6 +164,8 @@ struct bpf_call_arg_meta { >> bool pkt_access; >> int regno; >> int access_size; >> + s64 msize_smax_value; >> + u64 msize_umax_value; >> }; >> >> static DEFINE_MUTEX(bpf_verifier_lock); >> @@ -2027,6 +2029,14 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno, >> err = check_helper_mem_access(env, regno - 1, >> reg->umax_value, >> zero_size_allowed, meta); >> + >> + if (!err && !!meta) { > > Please drop !! in the above. > > Also what happens when > if (!tnum_is_const(reg->var_off)) > meta = NULL; > ? > it seems two new fields of meta will stay zero initialized > that later do_refine_retval_range() will set R0->umax_value = 0 > which seems incorrect. Thanks for catching this. In do_refine_retval_range(), if meta is NULL, the function should just return. Otherwise, a page fault will happen. > >> + /* remember the mem_size which may be used later >> + * to refine return values. >> + */ >> + meta->msize_smax_value = reg->smax_value; >> + meta->msize_umax_value = reg->umax_value; >> + } >> } >> >> return err; >> @@ -2333,6 +2343,21 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx) >> return 0; >> } >> >> +static void do_refine_retval_range(struct bpf_reg_state *regs, int ret_type, >> + int func_id, >> + struct bpf_call_arg_meta *meta) >> +{ >> + struct bpf_reg_state *ret_reg = ®s[BPF_REG_0]; >> + >> + if (ret_type != RET_INTEGER || >> + (func_id != BPF_FUNC_get_stack && >> + func_id != BPF_FUNC_probe_read_str)) >> + return; >> + >> + ret_reg->smax_value = meta->msize_smax_value; >> + ret_reg->umax_value = meta->msize_umax_value; >> +} >> + >> static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn_idx) >> { >> const struct bpf_func_proto *fn = NULL; >> @@ -2456,6 +2481,8 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn >> return -EINVAL; >> } >> >> + do_refine_retval_range(regs, fn->ret_type, func_id, &meta); >> + >> err = check_map_func_compatibility(env, meta.map_ptr, func_id); >> if (err) >> return err; >> -- >> 2.9.5 >>