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 <y...@fb.com> > --- > 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. > + /* 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 >