[PATCH AUTOSEL 4.19 03/17] evm: Fix RCU list related warnings

[Sasha Levin]

From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>

[ 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@xxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx> (RCU viewpoint)
Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 f0878d81dcef..d20f5792761c 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -215,7 +215,7 @@ 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 7f3f54d89a6e..e11d860fdce4 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -102,7 +102,7 @@ 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)
@@ -233,7 +233,7 @@ 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 77de71b7794c..f112ca593adc 100644
--- a/security/integrity/evm/evm_secfs.c
+++ b/security/integrity/evm/evm_secfs.c
@@ -237,7 +237,14 @@ 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