On 8 May 2018 at 23:14, Paolo Bonzini <[email protected]> wrote:
> From: Daniel P. Berrangé <[email protected]>
>
> 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)...
> + 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
