Re: [PATCH v4 3/5] security: Allow all LSMs to provide xattrs for inode_init_security hook

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On 11/17/2022 5:05 PM, Mimi Zohar wrote:
hOn Thu, 2022-11-10 at 10:46 +0100, Roberto Sassu wrote:
From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Currently, security_inode_init_security() supports only one LSM providing
an xattr and EVM calculating the HMAC on that xattr, plus other inode

Allow all LSMs to provide one or multiple xattrs, by extending the security
blob reservation mechanism. Introduce the new lbs_xattr field of the
lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
needs, and the LSM infrastructure knows how many xattr slots it should

Perhaps supporting per LSM multiple xattrs is a nice idea, but EVM
doesn't currently support it.  The LSM xattrs are hard coded in
evm_config_default_xattrnames[],  based on whether the LSM is
configured.  Additional security xattrs may be included in the
security.evm calculation, by extending the list via

EVM wouldn't notice whether it is the same LSM that provide multiple xattrs or multiple LSMs provided one xattr. As long as the xattr array contains consecutive xattrs, that would be fine. In the IMA/EVM test I included a test case where an LSM provides two xattrs (seems to work fine).

Dynamically allocate the xattrs array to be populated by LSMs with the
inode_init_security hook, and pass it to the latter instead of the
name/value/len triple.

Since the LSM infrastructure, at initialization time, updates the number of
the requested xattrs provided by each LSM with a corresponding offset in
the security blob (in this case the xattr array), it makes straightforward
for an LSM to access the right position in the xattr array.

There is still the issue that an LSM might not fill the xattr, even if it
requests it (legitimate case, for example it might have been loaded but not
initialized with a policy). Since users of the xattr array (e.g. the
initxattrs() callbacks) detect the end of the xattr array by checking if
the xattr name is NULL, not filling an xattr would cause those users to
stop scanning xattrs prematurely.

Solve that issue by introducing security_check_compact_xattrs(), which does
a basic check of the xattr array (if the xattr name is filled, the xattr
value should be too, and viceversa), and compacts the xattr array by
removing the holes.

An alternative solution would be to let users of the xattr array know the
number of elements of the xattr array, so that they don't have to check the
termination. However, this seems more invasive, compared to a simple move
of few array elements.

Finally, adapt both SELinux and Smack to use the new definition of the
inode_init_security hook, and to correctly fill the designated slots in the
xattr array.

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

diff --git a/security/security.c b/security/security.c
index a0e9b4ce2341..b62f192de6da 100644
--- a/security/security.c
+++ b/security/security.c
@@ -30,8 +30,6 @@
  #include <linux/msg.h>
  #include <net/flow.h>
-#define MAX_LSM_EVM_XATTR 2
  /* How many LSMs were built into the kernel? */
  #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
@@ -210,6 +208,7 @@ static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)
  	lsm_set_blob_size(&needed->lbs_msg_msg, &blob_sizes.lbs_msg_msg);
  	lsm_set_blob_size(&needed->lbs_superblock, &blob_sizes.lbs_superblock);
  	lsm_set_blob_size(&needed->lbs_task, &blob_sizes.lbs_task);
+	lsm_set_blob_size(&needed->lbs_xattr, &blob_sizes.lbs_xattr);
/* Prepare LSM for initialization. */
@@ -346,6 +345,7 @@ static void __init ordered_lsm_init(void)
  	init_debug("msg_msg blob size    = %d\n", blob_sizes.lbs_msg_msg);
  	init_debug("superblock blob size = %d\n", blob_sizes.lbs_superblock);
  	init_debug("task blob size       = %d\n", blob_sizes.lbs_task);
+	init_debug("xattr slots          = %d\n", blob_sizes.lbs_xattr);
  	 * Create any kmem_caches needed for blobs
@@ -1100,34 +1100,78 @@ static int security_initxattrs(struct inode *inode, const struct xattr *xattrs,
  	return 0;

+static int security_check_compact_xattrs(struct xattr *xattrs,
+					 int num_xattrs, int *checked_xattrs)

Perhaps the variable naming is off, making it difficult to read.   So
although this is a static function, which normally doesn't require a
comment, it's definitely needs one.

Ok, will improve it.

+	int i;
+	for (i = *checked_xattrs; i < num_xattrs; i++) {

If the number of "checked" xattrs was kept up to date, removing the
empty xattr gaps wouldn't require a loop.  Is the purpose of this loop
to support multiple per LSM xattrs?

An LSM might reserve one or more xattrs, but not set it/them (for example because it is not initialized). In this case, removing the gaps is needed for all subsequent LSMs.

+		if ((!xattrs[i].name && xattrs[i].value) ||
+		    (xattrs[i].name && !xattrs[i].value))
+			return -EINVAL;
+		if (!xattrs[i].name)
+			continue;
+		if (i == *checked_xattrs) {
+			(*checked_xattrs)++;
+			continue;
+		}
+		memcpy(xattrs + (*checked_xattrs)++, xattrs + i,
+		       sizeof(*xattrs));
+		memset(xattrs + i, 0, sizeof(*xattrs));
+	}
+	return 0;
  int security_inode_init_security(struct inode *inode, struct inode *dir,
  				 const struct qstr *qstr,
  				 const initxattrs initxattrs, void *fs_data)
-	struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
-	struct xattr *lsm_xattr, *evm_xattr, *xattr;
-	int ret = -EOPNOTSUPP;
+	struct security_hook_list *P;
+	struct xattr *new_xattrs;
+	struct xattr *xattr;
+	int ret = -EOPNOTSUPP, cur_xattrs = 0;
if (unlikely(IS_PRIVATE(inode)))
  		goto out_exit;
+ if (!blob_sizes.lbs_xattr)
+		goto out_exit;
  	if (!initxattrs ||
  	    (initxattrs == &security_initxattrs && !fs_data)) {
  		ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode,
-				    dir, qstr, NULL, NULL, NULL);
+				    dir, qstr, NULL);
  		goto out_exit;
-	memset(new_xattrs, 0, sizeof(new_xattrs));
-	lsm_xattr = new_xattrs;
-	ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
-						&lsm_xattr->name,
-						&lsm_xattr->value,
-						&lsm_xattr->value_len);
-	if (ret)
-		goto out;
+	/* Allocate +1 for EVM and +1 as terminator. */
+	new_xattrs = kcalloc(blob_sizes.lbs_xattr + 2, sizeof(*new_xattrs),
+			     GFP_NOFS);
+	if (!new_xattrs) {
+		ret = -ENOMEM;
+		goto out_exit;
+	}
+	hlist_for_each_entry(P, &security_hook_heads.inode_init_security,
+			     list) {
+		ret = P->hook.inode_init_security(inode, dir, qstr, new_xattrs);
+		if (ret && ret != -EOPNOTSUPP)
+			goto out;
+		if (ret == -EOPNOTSUPP)
+			continue;
+		ret = security_check_compact_xattrs(new_xattrs,
+						    blob_sizes.lbs_xattr,
+						    &cur_xattrs);

Defining a variable named "cur_xattrs" to indicate the number of xattrs
compressed is off.  Perhaps use cur_num_xattrs?   Similarly,
"checked_xattrs" should be num_checked_xattrs.  Or change the existing
num_xattrs to max_num_xattrs and rename checked_xattrs to num_xattrs.






+		if (ret < 0) {
+			ret = -ENOMEM;
+			goto out;
+		}
+	}
- evm_xattr = lsm_xattr + 1;
-	ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
+	ret = evm_inode_init_security(inode, new_xattrs,
+				      new_xattrs + cur_xattrs);
  	if (ret)
  		goto out;
  	ret = initxattrs(inode, new_xattrs, fs_data);
@@ -1142,6 +1186,7 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
+	kfree(new_xattrs);
  	if (initxattrs == &security_initxattrs)
  		return ret;

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