Re: [PATCH 1/1] fanotify: check permissions when creating file descriptor

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

On 24.04.2014 11:04, Jan Kara wrote:
> On Tue 22-04-14 16:07:47, Jan Kara wrote:
>> On Tue 22-04-14 15:50:26, Michael Kerrisk (man-pages) wrote:
>>> On Tue, Apr 22, 2014 at 3:40 PM, Jan Kara <jack@suse.cz> wrote:
>>>> On Sat 19-04-14 22:53:53, Heinrich Schuchardt wrote:
>>>>> When monitoring a directory or a mount with the fanotify API
>>>>> the call to fanotify_init checks,
>>>>>   * the process has cap_sys_admin capability
>>>>>
>>>>> The call to fanotify_mark checks,
>>>>>   * the process has read authorization for directory or mount
>>>>>
>>>>> A directory or mount may contain files for which the process
>>>>> has no read or write authorization.
>>>>> Yet when reading from the fanotify file descriptor, structures
>>>>> fanotify_event_metadata are returned, which contain a file
>>>>> descriptor for these files, and will allow to read or write.
>>>>>
>>>>> The patch adds an authorization check for read and write
>>>>> permission. In case of missing permission, reading from the
>>>>> fanotify file descriptor returns EACCES.
>>>>    OK, am I right you are concerned about a situation where fanotify group
>>>> descriptor is passed to an unpriviledged process which handles all the
>>>> incoming events? I'm asking because the permission checking can be
>>>> relatively expensive (think of acls) so we better do it for a reason.
>>>> I'd prefer to hear from Eric what the original intention regarding
>>>> permissions was...
>>>
>>> If I understand correctly, passing to an unprivileged process is the
>>> point. The point is I think that supposedly one only needs to
>>> CAP_SYS_ADMIN to use fanotify. However, once you have that capability,
>>> then you implicitly get the effect of CAP_DAC_READ_SEARCH and
>>> CAP_DAC_OVERRIDE as well.
>>    Ah, OK. Thanks for explanation. Then I'm OK with the patch. So feel free
>> to add:
>>
>> Reviewed-by: Jan Kara <jack@suse.cz>
>    Hum, when digging more around this code, I've found out that
> fanotify_mark() checks whether it has a read permission to a watched file
> when creating the mark (in fanotify_find_path()). So I don't think it's
> really worth it to recheck the permissions when creating a file .gnupg/secring.gpgdescriptor
> for the event. Sure it may be somewhat surprising that read fd is created
> after a process doesn't have access to the file anymore but OTOH it is
> similar to a situation where the process has opened the file long time ago.
>

fanotify_mark checks for the read authorization for the marked object,
not for the object for which the event occurs.

This means a listener may have read authorization for /home and mark 
this mount.
Afterwards, while you sign a git tag, it will receive a FAN_OPEN event
and use the file descriptor supplied in the event to overwrite your
/home/jankara/.gnupg/secring.pgp
though the file is chmod 600 and the listener is neither root nor you.

>
>>>>>   fs/notify/fanotify/fanotify_user.c | 20 +++++++++++++++-----
>>>>>   1 file changed, 15 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
>>>>> index 4e565c8..5d22a20 100644
>>>>> --- a/fs/notify/fanotify/fanotify_user.c
>>>>> +++ b/fs/notify/fanotify/fanotify_user.c
>>>>> @@ -62,6 +62,8 @@ static int create_fd(struct fsnotify_group *group,
>>>>>   {
>>>>>        int client_fd;
>>>>>        struct file *new_file;
>>>>> +     int mask;
>>>>> +     int ret;
>>>>>
>>>>>        pr_debug("%s: group=%p event=%p\n", __func__, group, event);
>>>>>
>>>>> @@ -75,11 +77,19 @@ static int create_fd(struct fsnotify_group *group,
>>>>>         */
>>>>>        /* it's possible this event was an overflow event.  in that case dentry and mnt
>>>>>         * are NULL;  That's fine, just don't call dentry open */
>>>>> -     if (event->path.dentry && event->path.mnt)
>>>>> -             new_file = dentry_open(&event->path,
>>>>> -                                    group->fanotify_data.f_flags | FMODE_NONOTIFY,
>>>>> -                                    current_cred());
>>>>> -     else
>>>>> +     if (event->path.dentry && event->path.mnt) {
>>>>> +             /* check permissions before granting access to file */
>>>>> +             mask = MAY_READ;
>>>>> +             if (group->fanotify_data.f_flags & (O_RDWR | O_WRONLY))
>>>>> +                     mask |= MAY_WRITE;
>>>>> +             ret = inode_permission(event->path.dentry->d_inode, mask);
>>>>> +             if (ret)
>>>>> +                     new_file = ERR_PTR(ret);
>>>>> +             else
>>>>> +                     new_file = dentry_open(&event->path,
>>>>> +                                            group->fanotify_data.f_flags | FMODE_NONOTIFY,
>>>>> +                                            current_cred());
>>>>> +     } else
>>>>>                new_file = ERR_PTR(-EOVERFLOW);
>>>>>        if (IS_ERR(new_file)) {
>>>>>                /*
>>>>> --
>>>>> 1.9.1
>>>>>
>>>> --
>>>> Jan Kara <jack@suse.cz>
>>>> SUSE Labs, CR
>>>
>>>
>>>
>>> --
>>> Michael Kerrisk
>>> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>>> Linux/UNIX System Programming Training: http://man7.org/training/
>> --
>> Jan Kara <jack@suse.cz>
>> SUSE Labs, CR