Messages in this thread | | | Date | Thu, 24 Apr 2014 15:54:06 +0200 | From | Heinrich Schuchardt <> | Subject | Re: [PATCH 1/1] fanotify: check permissions when creating file descriptor |
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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
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