[PATCH v10 3/5] overlayfs: add __get xattr method

[Mark Salyzyn <salyzyn@android.com>]

Because of the overlayfs getxattr recursion, the incoming inode fails
to update the selinux sid resulting in avc denials being reported
against a target context of u:object_r:unlabeled:s0.

Solution is to add a _get xattr method that calls the __vfs_getxattr
handler so that the context can be read in, rather than being denied
with an -EACCES when vfs_getxattr handler is called.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
---
v10 - added to patch series
---
 fs/overlayfs/inode.c     | 15 +++++++++++++++
 fs/overlayfs/overlayfs.h |  2 ++
 fs/overlayfs/super.c     | 18 ++++++++++++++++++
 3 files changed, 35 insertions(+)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 7663aeb85fa3..d3b53849615c 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -362,6 +362,21 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
 	return err;
 }
 
+int __ovl_xattr_get(struct dentry *dentry, struct inode *inode,
+		    const char *name, void *value, size_t size)
+{
+	ssize_t res;
+	const struct cred *old_cred;
+	struct dentry *realdentry =
+		ovl_i_dentry_upper(inode) ?: ovl_dentry_lower(dentry);
+
+	old_cred = ovl_override_creds(dentry->d_sb);
+	res = __vfs_getxattr(realdentry, d_inode(realdentry), name, value,
+			     size);
+	ovl_revert_creds(old_cred);
+	return res;
+}
+
 int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
 		  void *value, size_t size)
 {
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 6934bcf030f0..73a02a263fbc 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -357,6 +357,8 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
 		  const void *value, size_t size, int flags);
 int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
 		  void *value, size_t size);
+int __ovl_xattr_get(struct dentry *dentry, struct inode *inode,
+		    const char *name, void *value, size_t size);
 ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size);
 struct posix_acl *ovl_get_acl(struct inode *inode, int type);
 int ovl_update_time(struct inode *inode, struct timespec64 *ts, int flags);
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index b368e2e102fa..82e1130de206 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -859,6 +859,14 @@ ovl_posix_acl_xattr_get(const struct xattr_handler *handler,
 	return ovl_xattr_get(dentry, inode, handler->name, buffer, size);
 }
 
+static int __maybe_unused
+__ovl_posix_acl_xattr_get(const struct xattr_handler *handler,
+			  struct dentry *dentry, struct inode *inode,
+			  const char *name, void *buffer, size_t size)
+{
+	return __ovl_xattr_get(dentry, inode, handler->name, buffer, size);
+}
+
 static int __maybe_unused
 ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
 			struct dentry *dentry, struct inode *inode,
@@ -939,6 +947,13 @@ static int ovl_other_xattr_get(const struct xattr_handler *handler,
 	return ovl_xattr_get(dentry, inode, name, buffer, size);
 }
 
+static int __ovl_other_xattr_get(const struct xattr_handler *handler,
+				 struct dentry *dentry, struct inode *inode,
+				 const char *name, void *buffer, size_t size)
+{
+	return __ovl_xattr_get(dentry, inode, name, buffer, size);
+}
+
 static int ovl_other_xattr_set(const struct xattr_handler *handler,
 			       struct dentry *dentry, struct inode *inode,
 			       const char *name, const void *value,
@@ -952,6 +967,7 @@ ovl_posix_acl_access_xattr_handler = {
 	.name = XATTR_NAME_POSIX_ACL_ACCESS,
 	.flags = ACL_TYPE_ACCESS,
 	.get = ovl_posix_acl_xattr_get,
+	.__get = __ovl_posix_acl_xattr_get,
 	.set = ovl_posix_acl_xattr_set,
 };
 
@@ -960,6 +976,7 @@ ovl_posix_acl_default_xattr_handler = {
 	.name = XATTR_NAME_POSIX_ACL_DEFAULT,
 	.flags = ACL_TYPE_DEFAULT,
 	.get = ovl_posix_acl_xattr_get,
+	.__get = __ovl_posix_acl_xattr_get,
 	.set = ovl_posix_acl_xattr_set,
 };
 
@@ -972,6 +989,7 @@ static const struct xattr_handler ovl_own_xattr_handler = {
 static const struct xattr_handler ovl_other_xattr_handler = {
 	.prefix	= "", /* catch all */
 	.get = ovl_other_xattr_get,
+	.__get = __ovl_other_xattr_get,
 	.set = ovl_other_xattr_set,
 };
 
-- 
2.22.0.657.g960e92d24f-goog