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

[Mimi Zohar]

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, _value, _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 

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

[Thiago Jung Bauermann]

This patch actually implements the appraise_type=modsig option, allowing
IMA to read and verify modsig signatures

Signed-off-by: Thiago Jung Bauermann 
---
 security/integrity/ima/ima.h  |  17 +++--
 security/integrity/ima/ima_appraise.c | 119 --
 security/integrity/ima/ima_main.c |   7 +-
 3 files changed, 128 insertions(+), 15 deletions(-)

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index eb58af06566f..b082138461b3 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -157,7 +157,8 @@ void ima_init_template_list(void);
 
 static inline bool is_ima_sig(const struct evm_ima_xattr_data *xattr_value)
 {
-   return xattr_value && xattr_value->type == EVM_IMA_XATTR_DIGSIG;
+   return xattr_value && (xattr_value->type == EVM_IMA_XATTR_DIGSIG ||
+  xattr_value->type == IMA_MODSIG);
 }
 
 /*
@@ -243,9 +244,10 @@ int ima_policy_show(struct seq_file *m, void *v);
 #ifdef CONFIG_IMA_APPRAISE
 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);
 int ima_must_appraise(struct inode *inode, int mask, enum ima_hooks func);
 void ima_update_xattr(struct integrity_iint_cache *iint, struct file *file);
 enum integrity_status ima_get_cache_status(struct integrity_iint_cache *iint,
@@ -270,10 +272,11 @@ void ima_free_xattr_data(struct evm_ima_xattr_data *hdr);
 #else
 static inline int ima_appraise_measurement(enum ima_hooks func,
   struct integrity_iint_cache *iint,
-  struct file *file,
+  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)
+  struct evm_ima_xattr_data 
**xattr_value,
+  int *xattr_len, int opened)
 {
return INTEGRITY_UNKNOWN;
 }
diff --git a/security/integrity/ima/ima_appraise.c 
b/security/integrity/ima/ima_appraise.c
index 58e147049e98..108690741c1a 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -190,6 +190,45 @@ int ima_read_xattr(struct dentry *dentry,
return ret;
 }
 
+static int appraise_modsig(struct integrity_iint_cache *iint,
+  struct evm_ima_xattr_data *xattr_value,
+  int xattr_len)
+{
+   enum hash_algo algo;
+   const void *digest;
+   void *buf;
+   int rc, len;
+   u8 dig_len;
+
+   rc = ima_modsig_verify(INTEGRITY_KEYRING_IMA, xattr_value);
+   if (rc)
+   return rc;
+
+   /*
+* The signature is good. Now let's put the sig hash
+* into the iint cache so that it gets stored in the
+* measurement list.
+*/
+
+   rc = ima_get_modsig_hash(xattr_value, , , _len);
+   if (rc)
+   return rc;
+
+   len = sizeof(iint->ima_hash) + dig_len;
+   buf = krealloc(iint->ima_hash, len, GFP_NOFS);
+   if (!buf)
+   return -ENOMEM;
+
+   iint->ima_hash = buf;
+   iint->flags |= IMA_DIGSIG;
+   iint->ima_hash->algo = algo;
+   iint->ima_hash->length = dig_len;
+
+   memcpy(iint->ima_hash->digest, digest, dig_len);
+
+   return 0;
+}
+
 /*
  * ima_appraise_measurement - appraise file measurement
  *
@@ -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