[PATCH v2] evm: terminate and bound the evm_xattrs read buffer

[Pengpeng Hou]

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 to
simple_read_from_buffer().

Fixes: fa516b66a1bf ("EVM: Allow runtime modification of the set of verified xattrs")
Signed-off-by: Pengpeng Hou <pengpeng at iscas.ac.cn>
---
Changes since v1:
- add the Fixes tag
- replace sprintf() with snprintf()
- explicitly terminate the buffer instead of switching to kzalloc()

 security/integrity/evm/evm_secfs.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
index acd840461902..b7882a4ce9d0 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)
@@ -150,17 +150,18 @@ static ssize_t evm_read_xattrs(struct file *filp, char __user *buf,
 		mutex_unlock(&xattr_list_mutex);
 		return -ENOMEM;
 	}
+	temp[size] = '\0';
 
 	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)