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Integer overflows in Numeric #3652

clayton-shopify opened this Issue May 10, 2017 · 0 comments


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clayton-shopify commented May 10, 2017 reported the following:

Here we show the bug in the mruby with the build config from mruby-engine. So MRB_INT64 and MRB_WORD_BOXING is included in The MRB_INT_MAX of this build is 4611686018427387903 (2^62 - 1). The bug is also presented in other build configurations, but may triggered with different input.

$ mirb
> 2 ** 61 + 2 ** 61
 => 4.6116860184274e+18
> 2.0 ** 62
 => 4.6116860184274e+18
> 2 ** 62
 => -4611686018427387904

Note that the other operators (add, sub, ...) does check the overflow properly.
The root cause of this bug is very subtle. Here is how mruby implements num_pow:

static mrb_value
num_pow(mrb_state *mrb, mrb_value x)
  mrb_value y;
  mrb_float d, yv;

  mrb_get_args(mrb, "o", &y);
  yv = mrb_to_flo(mrb, y);
  d = pow(mrb_to_flo(mrb, x), yv);
  if (mrb_fixnum_p(x) && mrb_fixnum_p(y) && FIXABLE(d) && yv > 0 &&
      (d < 0 || (d > 0 && (mrb_int)d > 0)))
    return mrb_fixnum_value((mrb_int)d);
  return mrb_float_value(mrb, d);

The key part for overflow checking is FIXABLE, a C macro:

#define POSFIXABLE(f) ((f) <= MRB_INT_MAX)
#define NEGFIXABLE(f) ((f) >= MRB_INT_MIN)

Unfortunately, C macro expansion is just a typeless simple substitution. Since the variable d is a mrb_float, the constantsMRB_INT_MAX and MRB_INT_MIN are silently promoted from mrb_int to mrb_float as well. But the significand precision of double-precision floating-point is only 53 bits, so this check is passed. We can verify this behavior in python:

$ python
>>> float(2 ** 62) <= float(2 ** 62 - 1)

Another subtly failed overflow checking is (d > 0 && (mrb_int)d > 0). Because d is only 2^62, we can convert it to mrb_int without any problem. But when build with MRB_WORD_BOXING, the correspond slot in the mrb_value struct is a 63 bits bit-field:

typedef union mrb_value {
  union {
    void *p;
    struct {
      unsigned int i_flag : MRB_FIXNUM_SHIFT;
      mrb_int i : (MRB_INT_BIT - MRB_FIXNUM_SHIFT);
    // ...
  } value;
  unsigned long w;
} mrb_value;

So the value survives these two checks, but still cause an integer overflow.

When reviewing this bug, I found that in mruby, some functions pass int64_t to FIXABLE like int64_value, which has no problem. But there are many other places pass floating point to FIXABLE like flo_ceil, flo_floor, etc. All of them have the same problem as num_pow.

> (2.0 ** 62).to_i
 => -4611686018427387904
> (2.0 ** 62).ceil
 => -4611686018427387904
> (2.0 ** 62).floor
 => -4611686018427387904

If there is any business logic or boundary check rely on this, it would fail.

A possible fix is split the macro into two macros/functions, like FIXABLE_INT and FIXABLE_FLOAT. I can provide a patch if needed, but I think we may need someone (Matz?) who is familiar with the codebase of mruby to check the impact of this bug.

@matz matz closed this in 06c4933 May 26, 2017

matz added a commit that referenced this issue May 26, 2017

Add new range check macro FIXABLE_FLOAT(); ref #3652
When MRB_INT64, valid value range of mrb_int is bigger than double,
which only has 53 bits significant precision.
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