Re: [PATCH 1/1 v2] fanotify: check file flags passed in fanotify_init

[Heinrich Schuchardt]

On 13.05.2014 10:25, Michael Kerrisk (man-pages) wrote:
> Hello Heinrich
>
> On Mon, May 12, 2014 at 9:02 PM, Heinrich Schuchardt <xypron.glpk@xxxxxx> wrote:
> > 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@xxxxxx>
> > ---
> >   fs/notify/fanotify/fanotify_user.c | 25 +++++++++++++++++++++++++
> >   1 file changed, 25 insertions(+)
> >
> > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> > index d42220f..31b0de2 100644
> > --- a/fs/notify/fanotify/fanotify_user.c
> > +++ b/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;
>
> The above looks okay to me, but I'd be happier seeing an Ack from Jan or Eric.
>
> >   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;
> > +       }
> > +
>
> The 'switch' could just be replaced by:
>
> if ((event_f_flags & O_ACCMODE) == 3)
>          return -EINVAL;
>
> (But I'm not sure if some might prefer the idiom you have used.)
>
> Cheers,
>
> Michael

Using explicit numbers makes the code hard to read and analyze.
I do not intend to participate in the Obfuscated C Code Contest.

Best regards

Heinrich

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