ksmbd: add a permission check for FSCTL_SET_ZERO_DATA

commit 3320ba0 upstream.

FSCTL_SET_ZERO_DATA in smb2_ioctl() destroys file data via
ksmbd_vfs_zero_data() -> vfs_fallocate(PUNCH_HOLE/ZERO_RANGE) after
checking only the share-level KSMBD_TREE_CONN_FLAG_WRITABLE, with no
per-handle access check. A handle opened with only FILE_WRITE_ATTRIBUTES
still yields an FMODE_WRITE filp (FILE_WRITE_ATTRIBUTES is part of
FILE_WRITE_DESIRE_ACCESS_LE, so smb2_create_open_flags() opens it
O_WRONLY), so the vfs_fallocate FMODE_WRITE check does not stop it; only
the missing fp->daccess gate would. Reproduced on mainline 7.1-rc7 with
KASAN by an authenticated SMB client: a FILE_WRITE_ATTRIBUTES-only handle
zeroed 4096 bytes of file data it had no FILE_WRITE_DATA right to
(6/6; a FILE_READ_DATA-only handle was correctly denied).

This is the unfixed sibling of commit cc57232 ("ksmbd: fix FSCTL
permission bypass by adding a permission check for FSCTL_SET_SPARSE").
Because SET_ZERO_DATA writes data (not an attribute), require
FILE_WRITE_DATA.

Cc: stable@vger.kernel.org
Signed-off-by: Gil Portnoy <dddhkts1@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dhkts1

fs/smb/server/smb2pdu.c

@@ -8417,6 +8417,12 @@ int smb2_ioctl(struct ksmbd_work *work)
 				goto out;
 			}
 
+			if (!(fp->daccess & FILE_WRITE_DATA_LE)) {
+				ksmbd_fd_put(work, fp);
+				ret = -EACCES;
+				goto out;
+			}
+
 			ret = ksmbd_vfs_zero_data(work, fp, off, len);
 			ksmbd_fd_put(work, fp);
 			if (ret < 0)