Re: [Qemu-devel] [PULL 15/30] opts: don't silently truncate long option values

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, May 14, 2018 at 05:19:04PM +0100, Peter Maydell wrote:
> On 8 May 2018 at 23:14, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > From: Daniel P. Berrangé <berrange@redhat.com>
> >
> > The existing QemuOpts parsing code uses a fixed size 1024 byte buffer
> > for storing the option values. If a value exceeded this size it was
> > silently truncated and no error reported to the user. Long option values
> > is not a common scenario, but it is conceivable that they will happen.
> > eg if the user has a very deeply nested filesystem it would be possible
> > to come up with a disk path that was > 1024 bytes. Most of the time if
> > such data was silently truncated, the user would get an error about
> > opening a non-existant disk. If they're unlucky though, QEMU might use a
> > completely different disk image from another VM, which could be
> > considered a security issue. Another example program was in using the
> > -smbios command line arg with very large data blobs. In this case the
> > silent truncation will be providing semantically incorrect data to the
> > guest OS for SMBIOS tables.
> >
> > If the operating system didn't limit the user's argv when spawning QEMU,
> > the code should honour whatever length arguments were given without
> > imposing its own length restrictions. This patch thus changes the code
> > to use a heap allocated buffer for storing the values during parsing,
> > lifting the arbitrary length restriction.
> 
> Hi; Coverity doesn't like this change (CID1391003):
> 
> > --- a/util/qemu-option.c
> > +++ b/util/qemu-option.c
> > @@ -70,25 +70,37 @@ static const char *get_opt_name(const char *p, char **option, char delim)
> >   * delimiter is fixed to be comma which starts a new option. To specify an
> >   * option value that contains commas, double each comma.
> >   */
> > -const char *get_opt_value(char *buf, int buf_size, const char *p)
> > +const char *get_opt_value(const char *p, char **value)
> >  {
> > -    char *q;
> > +    size_t capacity = 0, length;
> > +    const char *offset;
> > +
> > +    *value = NULL;
> 
> Here we write to *value, so value must be non-NULL, and
> within the loop the only place we write to value it
> can't become NULL either (g_renew can't fail)...

Oh, real bug ! This should have been

   if (value) {
      *value = NULL;
   }

because multiboot.c passes in NULL for this parameter.

Unless we decide to rewrite multiboot.c to avoid that instead,
since all other callers pass non-NULL.

> 
> > +    while (1) {
> > +        offset = strchr(p, ',');
> > +        if (!offset) {
> > +            offset = p + strlen(p);
> > +        }
> >
> > -    q = buf;
> > -    while (*p != '\0') {
> > -        if (*p == ',') {
> > -            if (*(p + 1) != ',')
> > -                break;
> > -            p++;
> > +        length = offset - p;
> > +        if (*offset != '\0' && *(offset + 1) == ',') {
> > +            length++;
> > +        }
> > +        if (value) {
> 
> ...so this check for whether value is NULL can never be true.
> 
> > +            *value = g_renew(char, *value, capacity + length + 1);
> > +            strncpy(*value + capacity, p, length);
> > +            (*value)[capacity + length] = '\0';
> > +        }
> > +        capacity += length;
> > +        if (*offset == '\0' ||
> > +            *(offset + 1) != ',') {
> > +            break;
> >          }
> > -        if (q && (q - buf) < buf_size - 1)
> > -            *q++ = *p;
> > -        p++;
> > +
> > +        p += (offset - p) + 2;
> >      }
> > -    if (q)
> > -        *q = '\0';
> >
> > -    return p;
> > +    return offset;
> >  }
> >
> 
> thanks
> -- PMM
> 

Regards,
Daniel
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