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

[Heinrich Schuchardt <xypron.glpk@gmx.de>]

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@gmx.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@gmx.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

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