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

[Richard W.M. Jones]

On Mon, Nov 08, 2021 at 10:08:59PM +0200, Nir Soffer wrote:
> 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.

I agree too, it seems right here.

Guess it's one case where a proof checker could actually help :-)

Rich.

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