by Mimi Zohar

[permalink] [raw]

Subject: Re: [PATCH v5 17/18] ima: Implement support for module-style appended signatures

On Tue, 2017-10-17 at 22:53 -0200, Thiago Jung Bauermann wrote:

Below are a few additional comments.

> @@ -200,18 +239,28 @@ int ima_read_xattr(struct dentry *dentry,
> */
> int ima_appraise_measurement(enum ima_hooks func,
> struct integrity_iint_cache *iint,
> - struct file *file, const unsigned char *filename,
> - struct evm_ima_xattr_data *xattr_value,
> - int xattr_len, int opened)
> + struct file *file, const void *buf, loff_t size,
> + const unsigned char *filename,
> + struct evm_ima_xattr_data **xattr_value_,
> + int *xattr_len_, int opened)
> {
> static const char op[] = "appraise_data";
> const char *cause = "unknown";
> struct dentry *dentry = file_dentry(file);
> struct inode *inode = d_backing_inode(dentry);
> enum integrity_status status = INTEGRITY_UNKNOWN;
> - int rc = xattr_len, hash_start = 0;
> + struct evm_ima_xattr_data *xattr_value = *xattr_value_;
> + int xattr_len = *xattr_len_, rc = xattr_len, hash_start = 0;
> + bool appraising_modsig = false;
> +
> + if (iint->flags & IMA_MODSIG_ALLOWED &&
> + !ima_read_modsig(func, buf, size, &xattr_value, &xattr_len)) {
> + appraising_modsig = true;
> + rc = xattr_len;
> + }
>
> - if (!(inode->i_opflags & IOP_XATTR))
> + /* If not appraising a modsig, we need an xattr. */
> + if (!appraising_modsig && !(inode->i_opflags & IOP_XATTR))
> return INTEGRITY_UNKNOWN;
>
> if (rc <= 0) {
> @@ -235,6 +284,9 @@ int ima_appraise_measurement(enum ima_hooks func,
> case INTEGRITY_UNKNOWN:
> break;
> case INTEGRITY_NOXATTRS: /* No EVM protected xattrs. */
> + /* It's fine not to have xattrs when using a modsig. */
> + if (appraising_modsig)
> + break;
> case INTEGRITY_NOLABEL: /* No security.evm xattr. */
> cause = "missing-HMAC";
> goto out;
> @@ -242,6 +294,8 @@ int ima_appraise_measurement(enum ima_hooks func,
> cause = "invalid-HMAC";
> goto out;
> }
> +
> + retry:
> switch (xattr_value->type) {
> case IMA_XATTR_DIGEST_NG:
> /* first byte contains algorithm id */
> @@ -285,6 +339,61 @@ int ima_appraise_measurement(enum ima_hooks func,
> status = INTEGRITY_PASS;
> }
> break;
> + case IMA_MODSIG:
> + /*
> + * To avoid being tricked into an infinite loop, we don't allow
> + * a modsig stored in the xattr.
> + */
> + if (!appraising_modsig) {
> + status = INTEGRITY_UNKNOWN;
> + cause = "unknown-ima-data";
> + break;
> + }
> + rc = appraise_modsig(iint, xattr_value, xattr_len);
> + if (!rc) {
> + kfree(*xattr_value_);
> + *xattr_value_ = xattr_value;
> + *xattr_len_ = xattr_len;
> +
> + status = INTEGRITY_PASS;
> + break;
> + }
> +
> + ima_free_xattr_data(xattr_value);
> +
> + /*
> + * The appended signature failed verification. If there's a
> + * signature in the extended attribute, let's fall back to it.
> + */
> + if (inode->i_opflags & IOP_XATTR && *xattr_len_ != 0 &&
> + *xattr_len_ != -ENODATA) {

At this point, there was an appended signature verification failure.
If there isn't an xattr, for whatever reason, shouldn't we be
returning "invalid_signature" and "INTEGRITY_FAIL". If so, then the
above test could be simplified to check whether there is any data,
like this:

if ((inode->i_opflags & IOP_XATTR) && (*xattr_len_ > 0)) {

> + const char *modsig_cause = rc == -EOPNOTSUPP ?
> + "unknown" : "invalid-signature";

This can then be cleaned up as well.

> +
> + /* First, log that the modsig verification failed. */
> + integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode,
> + filename, op, modsig_cause, rc, 0);

I'm not sure that we want to audit intermediary signature verification
failures. Perhaps this audit message should be considered
"additional", meaning it is only emitted if the "integrity_audit" boot
command line option is enabled. Change the last field to 1 to
indicate it is an "additional" audit message.

> +
> + xattr_len = rc = *xattr_len_;
> + xattr_value = *xattr_value_;
> + appraising_modsig = false;
> +
> + if (rc > 0)

This test becomes redundant.

> + /* Process xattr contents. */
> + goto retry;
> +
> + /* Unexpected error reading xattr. */
> + status = INTEGRITY_UNKNOWN;
> + } else {
> + if (rc == -EOPNOTSUPP)
> + status = INTEGRITY_UNKNOWN;
> + else {
> + cause = "invalid-signature";
> + status = INTEGRITY_FAIL;
> + }
> + }
> + break;

I think the rest can be simplified to:
cause = "invalid-signature";
status = INTEGRITY_FAIL;

Mimi