Subject: Re: [Linux-ima-devel] [PATCH v6 02/10] ima: on soft reboot, restore the measurement list

From: Mimi Zohar <[email-masked]>
Date: Thu, 10 Nov 2016 08:12:32 -0500

On Tue, 2016-11-08 at 15:47 -0500, Mimi Zohar wrote:
> On Tue, 2016-11-08 at 21:46 +0200, Dmitry Kasatkin wrote:
> > On Fri, Oct 21, 2016 at 5:44 AM, Thiago Jung Bauermann

> > > +/* Restore the serialized binary measurement list without extending PCRs. */
> > > +int ima_restore_measurement_list(loff_t size, void *buf)
> > > +{
> > > +       struct binary_hdr_v1 {
> > > +               u32 pcr;
> > > +               u8 digest[TPM_DIGEST_SIZE];
> > > +               u32 template_name_len;
> > > +               char template_name[0];
> > > +       } __packed;
> > > +       char template_name[MAX_TEMPLATE_NAME_LEN];
> > > +
> > > +       struct binary_data_v1 {
> > > +               u32 template_data_size;
> > > +               char template_data[0];
> > > +       } __packed;
> > > +
> > > +       struct ima_kexec_hdr *khdr = buf;
> > > +       struct binary_hdr_v1 *hdr_v1;
> > > +       struct binary_data_v1 *data_v1;
> > > +
> > > +       void *bufp = buf + sizeof(*khdr);
> > > +       void *bufendp = buf + khdr->buffer_size;
> > > +       struct ima_template_entry *entry;
> > > +       struct ima_template_desc *template_desc;
> > > +       unsigned long count = 0;
> > > +       int ret = 0;
> > > +
> > > +       if (!buf || size < sizeof(*khdr))
> > > +               return 0;
> > > +
> > > +       if (khdr->version != 1) {
> > > +               pr_err("attempting to restore a incompatible measurement list");
> > > +               return 0;
> > > +       }
> > > +
> > > +       /*
> > > +        * ima kexec buffer prefix: version, buffer size, count
> > > +        * v1 format: pcr, digest, template-name-len, template-name,
> > > +        *            template-data-size, template-data
> > > +        */
> > > +       while ((bufp < bufendp) && (count++ < khdr->count)) {
> > > +               if (count > ULONG_MAX - 1) {
> > > +                       pr_err("attempting to restore too many measurements");
> > > +                       ret = -EINVAL;
> > > +               }
> > > +
> > > +               hdr_v1 = bufp;
> > > +               if ((hdr_v1->template_name_len >= MAX_TEMPLATE_NAME_LEN) ||
> > > +                   ((bufp + hdr_v1->template_name_len) > bufendp)) {
> > 
> > based on following code  template_name_len does not include header
> > (sizeof(*hdr_v1))?
> > If so the check is wrong???
> 
> Yes, good catch.  In addition, before assigning hdr_v1 there should be a
> size check as well. 
> 
> > 
> > > +                       pr_err("attempting to restore a template name \
> > > +                               that is too long\n");
> > > +                       ret = -EINVAL;
> > > +                       break;
> > > +               }
> > > +               bufp += sizeof(*hdr_v1);

This line should have been before the test above.

> > > +
> > > +               /* template name is not null terminated */
> > > +               memcpy(template_name, bufp, hdr_v1->template_name_len);
> > > +               template_name[hdr_v1->template_name_len] = 0;
> > > +
> > > +               if (strcmp(template_name, "ima") == 0) {
> > > +                       pr_err("attempting to restore an unsupported \
> > > +                               template \"%s\" failed\n", template_name);
> > > +                       ret = -EINVAL;
> > > +                       break;
> > > +               }
> > > +               data_v1 = bufp += (u_int8_t)hdr_v1->template_name_len;
> > > +
> > > +               /* get template format */
> > > +               template_desc = lookup_template_desc(template_name);
> > > +               if (!template_desc) {
> > > +                       pr_err("template \"%s\" not found\n", template_name);
> > > +                       ret = -EINVAL;
> > > +                       break;
> > > +               }
> > > +
> > > +               if (bufp > (bufendp - sizeof(data_v1->template_data_size))) {
> > > +                       pr_err("restoring the template data size failed\n");
> > > +                       ret = -EINVAL;
> > > +                       break;
> > > +               }
> > > +               bufp += (u_int8_t) sizeof(data_v1->template_data_size);
> > > +
> > > +               if (bufp > (bufendp - data_v1->template_data_size)) {
> > > +                       pr_err("restoring the template data failed\n");
> > > +                       ret = -EINVAL;
> > > +                       break;
> > > +               }
> > > +
> > 
> > It looks like a similar problem... sizeof(struct binary_data_v1) is
> > missing in the check...
> 
> Following the template name, is the template data length and the
> template data. 
> 
> > > +               ret = ima_restore_template_data(template_desc,
> > > +                                               data_v1->template_data,
> > > +                                               data_v1->template_data_size,
> > > +                                               &entry);
> > > +               if (ret < 0)
> > > +                       break;
> > > +
> > > +               memcpy(entry->digest, hdr_v1->digest, TPM_DIGEST_SIZE);
> > > +               entry->pcr = hdr_v1->pcr;
> > > +               ret = ima_restore_measurement_entry(entry);
> > > +               if (ret < 0)
> > > +                       break;
> > > +
> > > +               bufp += data_v1->template_data_size;
> > > +       }
> > > +       return ret;
> > > +}
> > > --
> > > 2.7.4
> > >
> > 
> > In overall it is a bit hard to read this function somehow..
> 
> Ok, I'll see if there is any way of simplifying/cleaning up walking the
> measurement list some more.

I moved some code around so that the bufp pointer update is immediately
after the  buffer overflow tests and moved one check outside the while
loop.  I tried defining a buffer overflow macro, but that didn't seem to
help.

The updated patches are available in the next-kexec-restore branch of:
git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity.git

Mimi