[Libguestfs] [libnbd PATCH] common/utils/vector: Better overflow handling

[Nir Soffer]

On Mon, Nov 8, 2021 at 9:56 PM Eric Blake <eblake at redhat.com> wrote:
>
> Check newcap * itemsize for overflow prior to calling realloc, so that
> we don't accidentally truncate an existing array.  Set errno to ENOMEM
> on all failure paths, rather than leaving it indeterminate on
> overflow.  The patch works by assuming a prerequisite that
> v->cap*itemsize is always smaller than size_t.
>
> Fixes: 985dfa72ae (common/utils/vector.c: Optimize vector append)
> ---
>
> Unless you see problems in this, I'll push this and also port it to
> nbdkit.
>
>  common/utils/vector.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/common/utils/vector.c b/common/utils/vector.c
> index a4b43ce..c37f0c3 100644
> --- a/common/utils/vector.c
> +++ b/common/utils/vector.c
> @@ -1,5 +1,5 @@
>  /* nbdkit
> - * Copyright (C) 2018-2020 Red Hat Inc.
> + * Copyright (C) 2018-2021 Red Hat Inc.
>   *
>   * Redistribution and use in source and binary forms, with or without
>   * modification, are permitted provided that the following conditions are
> @@ -44,12 +44,14 @@ generic_vector_reserve (struct generic_vector *v, size_t n, size_t itemsize)
>    size_t reqcap, newcap;
>
>    reqcap = v->cap + n;
> -  if (reqcap < v->cap)
> +  if (reqcap * itemsize < v->cap * itemsize) {

Just to make sure I understand this correctly:
v->cap * itemsize must be valid value since this is the current
allocation, computed in a previous call to generic_vector_reserve...

> +    errno = ENOMEM;
>      return -1; /* overflow */
> +  }
>
>    newcap = (v->cap * 3 + 1) / 2;
>
> -  if (newcap < reqcap)
> +  if (newcap * itemsize < reqcap * itemsize)

So reqcap * itemsize is valid because we passed the check above.

>      newcap = reqcap;
>
>    newptr = realloc (v->ptr, newcap * itemsize);

Looks right to me.

Nir