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

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Message-ID: <CAKgNAkjrrPubqOq=m2UXC-GkhvAReSwCxRQydDeGtPWYrMbQVQ@mail.gmail.com>
Date:	Thu, 24 Apr 2014 17:05:45 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Jan Kara <jack@...e.cz>, Eric Paris <eparis@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] fanotify: check permissions when creating file descriptor

On Thu, Apr 24, 2014 at 3:54 PM, Heinrich Schuchardt <xypron.glpk@....de> wrote:
> 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@...e.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@...e.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

Sweet!



-- 
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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