linux-kernel - Re: [PATCH 1/1 v2] fanotify: check file flags passed in fanotify

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Message-ID: <CAKgNAkjd=faKp8-QroLHXDOk00tKYxvAzm0xZV4PjYLNO+3_Ew@mail.gmail.com>
Date:	Tue, 13 May 2014 10:25:14 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Eric Paris <eparis@...hat.com>, Jan Kara <jack@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1 v2] fanotify: check file flags passed in fanotify_init

Hello Heinrich

On Mon, May 12, 2014 at 9:02 PM, Heinrich Schuchardt <xypron.glpk@....de> 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@....de>
> ---
>  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




-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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