[PATCH] ksmbd: vfs: fix race on m_flags in vfs_cache

[Qianchang Zhao <pioooooooooip@gmail.com>]

ksmbd maintains delete-on-close and pending-delete state in
ksmbd_inode->m_flags. In vfs_cache.c this field is accessed under
inconsistent locking: some paths read and modify m_flags under
ci->m_lock while others do so without taking the lock at all.

Examples:

 - ksmbd_query_inode_status() and __ksmbd_inode_close() use
   ci->m_lock when checking or updating m_flags.
 - ksmbd_inode_pending_delete(), ksmbd_set_inode_pending_delete(),
   ksmbd_clear_inode_pending_delete() and ksmbd_fd_set_delete_on_close()
   used to read and modify m_flags without ci->m_lock.

This creates a potential data race on m_flags when multiple threads
open, close and delete the same file concurrently. In the worst case
delete-on-close and pending-delete bits can be lost or observed in an
inconsistent state, leading to confusing delete semantics (files that
stay on disk after delete-on-close, or files that disappear while still
in use).

Fix it by:

 - Making ksmbd_query_inode_status() look at m_flags under ci->m_lock
   after dropping inode_hash_lock.
 - Adding ci->m_lock protection to all helpers that read or modify
   m_flags (ksmbd_inode_pending_delete(), ksmbd_set_inode_pending_delete(),
   ksmbd_clear_inode_pending_delete(), ksmbd_fd_set_delete_on_close()).
 - Keeping the existing ci->m_lock protection in __ksmbd_inode_close(),
   and moving the actual unlink/xattr removal outside the lock.

This unifies the locking around m_flags and removes the data race while
preserving the existing delete-on-close behaviour.

Reported-by: Qianchang Zhao <pioooooooooip@gmail.com>
Reported-by: Zhitong Liu <liuzhitong1993@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Qianchang Zhao <pioooooooooip@gmail.com>
---
 fs/smb/server/vfs_cache.c | 114 +++++++++++++++++++++++++++++---------
 1 file changed, 87 insertions(+), 27 deletions(-)

diff --git a/fs/smb/server/vfs_cache.c b/fs/smb/server/vfs_cache.c
index dfed6fce8..b44e0d618 100644
--- a/fs/smb/server/vfs_cache.c
+++ b/fs/smb/server/vfs_cache.c
@@ -112,40 +112,88 @@ int ksmbd_query_inode_status(struct dentry *dentry)
 
 	read_lock(&inode_hash_lock);
 	ci = __ksmbd_inode_lookup(dentry);
-	if (ci) {
-		ret = KSMBD_INODE_STATUS_OK;
-		if (ci->m_flags & (S_DEL_PENDING | S_DEL_ON_CLS))
-			ret = KSMBD_INODE_STATUS_PENDING_DELETE;
-		atomic_dec(&ci->m_count);
-	}
 	read_unlock(&inode_hash_lock);
+	if (!ci)
+		return ret;
+	down_read(&ci->m_lock);
+	if (ci->m_flags & (S_DEL_PENDING | S_DEL_ON_CLS))
+		ret = KSMBD_INODE_STATUS_PENDING_DELETE;
+	else
+		ret = KSMBD_INODE_STATUS_OK;
+	up_read(&ci->m_lock);
+
+	atomic_dec(&ci->m_count);
 	return ret;
 }
 
 bool ksmbd_inode_pending_delete(struct ksmbd_file *fp)
 {
-	return (fp->f_ci->m_flags & (S_DEL_PENDING | S_DEL_ON_CLS));
+	struct ksmbd_inode *ci;
+	bool ret;
+
+	if (!fp)
+		return false;
+
+	ci = fp->f_ci;
+	if (!ci)
+		return false;
+
+	down_read(&ci->m_lock);
+	ret = ci->m_flags & (S_DEL_PENDING | S_DEL_ON_CLS);
+	up_read(&ci->m_lock);
+
+	return ret;
 }
 
 void ksmbd_set_inode_pending_delete(struct ksmbd_file *fp)
 {
-	fp->f_ci->m_flags |= S_DEL_PENDING;
+	struct ksmbd_inode *ci;
+
+	if (!fp)
+		return;
+
+	ci = fp->f_ci;
+	if (!ci)
+		return;
+
+	down_write(&ci->m_lock);
+	ci->m_flags |= S_DEL_PENDING;
+	up_write(&ci->m_lock);
 }
 
 void ksmbd_clear_inode_pending_delete(struct ksmbd_file *fp)
 {
-	fp->f_ci->m_flags &= ~S_DEL_PENDING;
+	struct ksmbd_inode *ci;
+
+	if (!fp)
+		return;
+
+	ci = fp->f_ci;
+	if (!ci)
+		return;
+
+	down_write(&ci->m_lock);
+	ci->m_flags &= ~S_DEL_PENDING;
+	up_write(&ci->m_lock);
 }
 
-void ksmbd_fd_set_delete_on_close(struct ksmbd_file *fp,
-				  int file_info)
+void ksmbd_fd_set_delete_on_close(struct ksmbd_file *fp, int file_info)
 {
-	if (ksmbd_stream_fd(fp)) {
-		fp->f_ci->m_flags |= S_DEL_ON_CLS_STREAM;
+	struct ksmbd_inode *ci;
+
+	if (!fp)
+		return;
+
+	ci = fp->f_ci;
+	if (!ci)
 		return;
-	}
 
-	fp->f_ci->m_flags |= S_DEL_ON_CLS;
+	down_write(&ci->m_lock);
+	if (ksmbd_stream_fd(fp))
+		ci->m_flags |= S_DEL_ON_CLS_STREAM;
+	else
+		ci->m_flags |= S_DEL_ON_CLS;
+	up_write(&ci->m_lock);
 }
 
 static void ksmbd_inode_hash(struct ksmbd_inode *ci)
@@ -255,29 +303,41 @@ static void __ksmbd_inode_close(struct ksmbd_file *fp)
 	struct ksmbd_inode *ci = fp->f_ci;
 	int err;
 	struct file *filp;
+	bool remove_stream_xattr = false;
+	bool do_unlink = false;
 
 	filp = fp->filp;
-	if (ksmbd_stream_fd(fp) && (ci->m_flags & S_DEL_ON_CLS_STREAM)) {
-		ci->m_flags &= ~S_DEL_ON_CLS_STREAM;
-		err = ksmbd_vfs_remove_xattr(file_mnt_idmap(filp),
-					     &filp->f_path,
-					     fp->stream.name,
-					     true);
-		if (err)
-			pr_err("remove xattr failed : %s\n",
-			       fp->stream.name);
+
+	if (ksmbd_stream_fd(fp)) {
+		down_write(&ci->m_lock);
+		if (ci->m_flags & S_DEL_ON_CLS_STREAM) {
+			ci->m_flags &= ~S_DEL_ON_CLS_STREAM;
+			remove_stream_xattr = true;
+		}
+		up_write(&ci->m_lock);
+
+		if (remove_stream_xattr) {
+			err = ksmbd_vfs_remove_xattr(file_mnt_idmap(filp),
+						     &filp->f_path,
+						     fp->stream.name,
+						     true);
+			if (err)
+				pr_err("remove xattr failed : %s\n",
+				       fp->stream.name);
+		}
 	}
 
 	if (atomic_dec_and_test(&ci->m_count)) {
 		down_write(&ci->m_lock);
 		if (ci->m_flags & (S_DEL_ON_CLS | S_DEL_PENDING)) {
 			ci->m_flags &= ~(S_DEL_ON_CLS | S_DEL_PENDING);
-			up_write(&ci->m_lock);
-			ksmbd_vfs_unlink(filp);
-			down_write(&ci->m_lock);
+			do_unlink = true;
 		}
 		up_write(&ci->m_lock);
 
+		if (do_unlink)
+			ksmbd_vfs_unlink(filp);
+
 		ksmbd_inode_free(ci);
 	}
 }
-- 
2.34.1