From: Mimi Zohar [mailto:zohar@linux.ibm.com]
Sent: Friday, July 30, 2021 2:40 PM
Hi Roberto,

On Fri, 2021-07-30 at 07:16 +0000, Roberto Sassu wrote:

quoted

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From: Mimi Zohar [mailto:zohar@linux.ibm.com]
Sent: Thursday, July 29, 2021 11:21 PM

On Mon, 2021-07-26 at 18:36 +0200, Roberto Sassu wrote:

quoted

/*
+ * digest_list_read: read and parse the digest list from the path
+ */
+static ssize_t digest_list_read(char *path, enum ops op)
+{
+       void *data = NULL;
+       char *datap;
+       size_t size;
+       u8 actions = 0;
+       struct file *file;
+       char event_name[NAME_MAX + 9 + 1];
+       u8 digest[IMA_MAX_DIGEST_SIZE] = { 0 };
+       enum hash_algo algo;
+       int rc, pathlen = strlen(path);
+
+       /* Remove \n. */
+       datap = path;
+       strsep(&datap, "\n");
+
+       file = filp_open(path, O_RDONLY, 0);
+       if (IS_ERR(file)) {
+               pr_err("unable to open file: %s (%ld)", path, PTR_ERR(file));
+               return PTR_ERR(file);
+       }
+
+       rc = kernel_read_file(file, 0, &data, INT_MAX, NULL,
+                             READING_DIGEST_LIST);
+       if (rc < 0) {
+               pr_err("unable to read file: %s (%d)", path, rc);
+               goto out;
+       }
+
+       size = rc;
+
+       snprintf(event_name, sizeof(event_name), "%s_file_%s",
+                op == DIGEST_LIST_ADD ? "add" : "del",
+                file_dentry(file)->d_name.name);
+
+       rc = ima_measure_critical_data("diglim", event_name, data, size,

false,

quoted

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+                                      digest, sizeof(digest));
+       if (rc < 0 && rc != -EEXIST)
+               goto out_vfree;

The digest lists could easily be measured while reading the digest list
file above in kernel_read_file().  What makes it "critical-data"?  In
the SELinux case, the in memory SELinux policy is being measured and
re-measured to make sure it hasn't been modified.  Is the digest list
file data being measured more than once?

Hi Mimi

yes, the digest lists can be measured with kernel_read_file().
I didn't send the change yet, but I added a DIGEST_LIST_CHECK
hook mapped to READING_DIGEST_LIST, so that digest lists
can be easily measured or appraised.

The point was that the digest of the digest list must be always
calculated, as it is added to the hash table. Instead of duplicating
the code, I preferred to use ima_measure_critical_data().

The advantage is also that, if the use case is to just measure
digest lists, ima_measure_critical_data() could do both at the
same time.

Digest lists can be seen as "critical data" in the sense that
they can affect the security decision on whether to grant
access to a file or not, assuming that an appropriate rule is
added in the IMA policy.

Of course the integrity of files containing the digest lists is
important, but that doesn't make them "critical data".  If the
integrity of these files is important, then the digest lists not only
need to be measured, but they need to be signed and the resulting
signature verified.  Without signature verification, there is no basis
on which to trust the digest lists data.

The reason of storing the actions performed by IMA on the
digest lists helps to determine for which purpose they can be
used. If digest lists are used only for measurement purpose,
it should be sufficient that digest lists are measured. The
same applies for appraisal.

Adding the kernel_read_file() "READING_DIGEST_LIST" support in IMA does
not seem to be optional.  IMA would then be calculating the digest list
file hash twice, once in kernel_read_file() and then, again, in
ima_measure_critical_data().

I didn't include also this part: I retrieve the integrity_iint_cache for
the opened file descriptor and I get the flags from there. If the
IMA_MEASURED flag is set, it is not necessary to call also
ima_measure_critical_data().

quoted

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I understand that with your changes to ima_measure_critical_data(),
which are now in next-integrity-testing branch, allow IMA to calculate
the file data hash.

Yes, correct. But actually there is another useful use case.
If digest lists are not in the format supported by the kernel,
the user space parser has to convert them before uploading
them to the kernel.

ima_measure_critical_data() would in this case measure
the converted digest list (it is written directly, without
sending the file path). It is easier to attest the result,
instead of determining whether the user space parser
produced the expected result (by checking the files it
read).

The application to properly convert the digest list file data into the
appropriate format would need to be trusted.  I'm concerned that not
requiring the converted data to be signed and the signature verified is
introducing a new integrity gap.  Perhaps between an LSM policy,
limiting which files may be read by the application, and an IMA policy,
requiring all files read by this application to be measured and the
signature verified, this integrity gap could be averted.

It is the weakest point in the chain, yes. Relying on existing LSMs
didn't seem to me a good idea, as:
- a new policy must be installed
- we must be sure that the policy is really enforced
- we need to support different LSMs (SELinux for Fedora,
  Apparmor for SUSE)
- there might be no LSM we can rely on

For these reasons, I developed a new LSM. Its purpose is to
identify the user space parser and for each file it opens, ensure
that the file has been measured or appraised by IMA. If one of
these actions are missing, it will not be set in the digest list the
user space parser uploads to the kernel (which means that IMA
will ignore the digest list for that specific action).

"critical data", in this context, should probably be used for verifying
the in memory file digests and other state information haven't been
compromised.

Actually, this is what we are doing currently. To keep the
implementation simple, once the file or the buffer are uploaded
to the kernel, they will not be modified, just accessed through
the indexes.

I could send the second part of the patch set, so that it becomes
more clear how digest lists are used by IMA and how the
integrity gap is filled.

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

thanks,

Mimi