fanotify: check file flags passed in fanotify_init

Without this patch fanotify_init does not validate the value passed in
event_f_flags.

When a fanotify event is read from the fanotify file descriptor a new
file descriptor is created where file.f_flags = event_f_flags.

Internal and external open flags are stored together in field f_flags of
struct file.  Hence, an application might create file descriptors with
internal flags like FMODE_EXEC, FMODE_NOCMTIME set.

Jan Kara and Eric Paris both aggreed that this is a bug and the value of
event_f_flags should be checked:
  https://lkml.org/lkml/2014/4/29/522
  https://lkml.org/lkml/2014/4/29/539

This updated patch version considers the comments by Michael Kerrisk in
  https://lkml.org/lkml/2014/5/4/10

With the patch the value of event_f_flags is checked.
When specifying an invalid value error EINVAL is returned.

Internal flags are disallowed.

File creation flags are disallowed:
O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TRUNC, and O_TTY_INIT.

Flags which do not make sense with fanotify are disallowed:
__O_TMPFILE, O_PATH, FASYNC, and O_DIRECT.

This leaves us with the following allowed values:

O_RDONLY, O_WRONLY, O_RDWR are basic functionality. The are stored in the
bits given by O_ACCMODE.

O_APPEND is working as expected. The value might be useful in a logging
application which appends the current status each time the log is opened.

O_LARGEFILE is needed for files exceeding 4GB on 32bit systems.

O_NONBLOCK may be useful when monitoring slow devices like tapes.

O_NDELAY is equal to O_NONBLOCK except for platform parisc.
To avoid code breaking on parisc either both flags should be
allowed or none. The patch allows both.

__O_SYNC and O_DSYNC may be used to avoid data loss on power disruption.

O_NOATIME may be useful to reduce disk activity.

O_CLOEXEC may be useful, if separate processes shall be used to scan files.

Once this patch is accepted, the fanotify_init.2 manpage has to be updated.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/notify/fanotify/fanotify_user.c

@@ -25,6 +25,19 @@
 #define FANOTIFY_DEFAULT_MAX_MARKS	8192
 #define FANOTIFY_DEFAULT_MAX_LISTENERS	128
 
+/*
+ * All flags that may be specified in parameter event_f_flags of fanotify_init.
+ *
+ * Internal and external open flags are stored together in field f_flags of
+ * struct file. Only external open flags shall be allowed in event_f_flags.
+ * Internal flags like FMODE_NONOTIFY, FMODE_EXEC, FMODE_NOCMTIME shall be
+ * excluded.
+ */
+#define	FANOTIFY_INIT_ALL_EVENT_F_BITS				( \
+		O_ACCMODE	| O_APPEND	| O_NONBLOCK	| \
+		__O_SYNC	| O_DSYNC	| O_CLOEXEC     | \
+		O_LARGEFILE	| O_NOATIME	)
+
 extern const struct fsnotify_ops fanotify_fsnotify_ops;
 
 static struct kmem_cache *fanotify_mark_cache __read_mostly;
@@ -669,6 +682,18 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 	if (flags & ~FAN_ALL_INIT_FLAGS)
 		return -EINVAL;
 
+	if (event_f_flags & ~FANOTIFY_INIT_ALL_EVENT_F_BITS)
+		return -EINVAL;
+
+	switch (event_f_flags & O_ACCMODE) {
+	case O_RDONLY:
+	case O_RDWR:
+	case O_WRONLY:
+		break;
+	default:
+		return -EINVAL;
+	}
+
 	user = get_current_user();
 	if (atomic_read(&user->fanotify_listeners) > FANOTIFY_DEFAULT_MAX_LISTENERS) {
 		free_uid(user);