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

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Date:	Tue, 22 Apr 2014 15:50:26 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Heinrich Schuchardt <xypron.glpk@....de>,
	Eric Paris <eparis@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] fanotify: check permissions when creating file descriptor

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.

Cheers,

Michael


>>  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@...e.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/
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