[PATCH v3] evm: terminate and bound the evm_xattrs read buffer

[Pengpeng Hou <pengpeng@iscas.ac.cn>]

evm_read_xattrs() allocates size + 1 bytes, fills them from the list of
enabled xattrs, and then passes strlen(temp) to
simple_read_from_buffer(). When no configured xattrs are enabled, the
fill loop stores nothing and temp[0] remains uninitialized, so strlen()
reads beyond initialized memory.

Explicitly terminate the buffer after allocation, use snprintf() for
each formatted line, and pass the accumulated length, without risk of
truncation, to simple_read_from_buffer().

Fixes: fa516b66a1bf ("EVM: Allow runtime modification of the set of verified xattrs")
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
---
Changes since v2:
- adjust the changelog wording to mention why the accumulated length is
  safe
- add the blank line after the allocation error path
- add a comment explaining why snprintf() cannot truncate in the fill loop

diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
index acd840461902..4baf5e23bc97 100644
--- a/security/integrity/evm/evm_secfs.c
+++ b/security/integrity/evm/evm_secfs.c
@@ -127,8 +127,8 @@ static ssize_t evm_read_xattrs(struct file *filp, char __user *buf,
 			       size_t count, loff_t *ppos)
 {
 	char *temp;
-	int offset = 0;
-	ssize_t rc, size = 0;
+	size_t offset = 0, size = 0;
+	ssize_t rc;
 	struct xattr_list *xattr;
 
 	if (*ppos != 0)
@@ -151,16 +151,22 @@ static ssize_t evm_read_xattrs(struct file *filp, char __user *buf,
 		return -ENOMEM;
 	}
 
+	temp[size] = '\0';
+
+	/*
+	 * No truncation possible: size is computed over the same enabled
+	 * xattrs under xattr_list_mutex, so offset never exceeds size.
+	 */
 	list_for_each_entry(xattr, &evm_config_xattrnames, list) {
 		if (!xattr->enabled)
 			continue;
 
-		sprintf(temp + offset, "%s\n", xattr->name);
-		offset += strlen(xattr->name) + 1;
+		offset += snprintf(temp + offset, size + 1 - offset, "%s\n",
+				   xattr->name);
 	}
 
 	mutex_unlock(&xattr_list_mutex);
-	rc = simple_read_from_buffer(buf, count, ppos, temp, strlen(temp));
+	rc = simple_read_from_buffer(buf, count, ppos, temp, offset);
 
 	kfree(temp);
 
-- 
2.50.1 (Apple Git-155)