[PATCH AUTOSEL 5.6 04/47] evm: Fix RCU list related warnings

Sasha Levin
2020-05-28T12:10:59Z

From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>

[ Upstream commit 770f60586d2af0590be263f55fd079226313922c ]

This patch fixes the following warning and few other instances of
traversal of evm_config_xattrnames list:

[ 32.848432] =============================
[ 32.848707] WARNING: suspicious RCU usage
[ 32.848966] 5.7.0-rc1-00006-ga8d5875ce5f0b #1 Not tainted
[ 32.849308] -----------------------------
[ 32.849567] security/integrity/evm/evm_main.c:231 RCU-list traversed in non-reader section!!

Since entries are only added to the list and never deleted, use
list_for_each_entry_lockless() instead of list_for_each_entry_rcu for
traversing the list. Also, add a relevant comment in evm_secfs.c to
indicate this fact.

Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org> (RCU viewpoint)
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
security/integrity/evm/evm_crypto.c | 2 +-
security/integrity/evm/evm_main.c | 4 ++--
security/integrity/evm/evm_secfs.c | 9 ++++++++-
3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index cc826c2767a3..fbc2ee6d46fc 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -209,7 +209,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
data->hdr.length = crypto_shash_digestsize(desc->tfm);

error = -ENODATA;
- list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
+ list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
bool is_ima = false;

if (strcmp(xattr->name, XATTR_NAME_IMA) == 0)
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index f9a81b187fae..a2c393385db0 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -99,7 +99,7 @@ static int evm_find_protected_xattrs(struct dentry *dentry)
if (!(inode->i_opflags & IOP_XATTR))
return -EOPNOTSUPP;

- list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
+ list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
error = __vfs_getxattr(dentry, inode, xattr->name, NULL, 0);
if (error < 0) {
if (error == -ENODATA)
@@ -230,7 +230,7 @@ static int evm_protected_xattr(const char *req_xattr_name)
struct xattr_list *xattr;

namelen = strlen(req_xattr_name);
- list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
+ list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
if ((strlen(xattr->name) == namelen)
&& (strncmp(req_xattr_name, xattr->name, namelen) == 0)) {
found = 1;
diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
index c11c1f7b3ddd..0f37ef27268d 100644
--- a/security/integrity/evm/evm_secfs.c
+++ b/security/integrity/evm/evm_secfs.c
@@ -234,7 +234,14 @@ static ssize_t evm_write_xattrs(struct file *file, const char __user *buf,
goto out;
}

- /* Guard against races in evm_read_xattrs */
+ /*
+ * xattr_list_mutex guards against races in evm_read_xattrs().
+ * Entries are only added to the evm_config_xattrnames list
+ * and never deleted. Therefore, the list is traversed
+ * using list_for_each_entry_lockless() without holding
+ * the mutex in evm_calc_hmac_or_hash(), evm_find_protected_xattrs()
+ * and evm_protected_xattr().
+ */
mutex_lock(&xattr_list_mutex);
list_for_each_entry(tmp, &evm_config_xattrnames, list) {
if (strcmp(xattr->name, tmp->name) == 0) {
--
2.25.1