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Date:   Mon, 16 May 2022 16:53:08 -0400
From:   Richard Guy Briggs <rgb@redhat.com>
To:     Jan Kara <jack@suse.cz>
Cc:     Paul Moore <paul@paul-moore.com>,
        Linux-Audit Mailing List <linux-audit@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, Eric Paris <eparis@parisplace.org>,
        Steve Grubb <sgrubb@redhat.com>,
        Amir Goldstein <amir73il@gmail.com>
Subject: Re: [PATCH v2 2/3] fanotify: define struct members to hold response
 decision context
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On 2022-05-09 10:54, Jan Kara wrote:
> On Fri 06-05-22 14:46:49, Richard Guy Briggs wrote:
> > On 2022-05-05 16:44, Jan Kara wrote:
> > > On Tue 03-05-22 21:33:35, Richard Guy Briggs wrote:
> > > > On 2022-05-02 20:16, Paul Moore wrote:
> > > > > On Thu, Apr 28, 2022 at 8:45 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> > > > > > This patch adds 2 structure members to the response returned from user
> > > > > > space on a permission event. The first field is 16 bits for the context
> > > > > > type.  The context type will describe what the meaning is of the second
> > > > > > field. The default is none. The patch defines one additional context
> > > > > > type which means that the second field is a 32-bit rule number. This
> > > > > > will allow for the creation of other context types in the future if
> > > > > > other users of the API identify different needs.  The second field size
> > > > > > is defined by the context type and can be used to pass along the data
> > > > > > described by the context.
> > > > > >
> > > > > > To support this, there is a macro for user space to check that the data
> > > > > > being sent is valid. Of course, without this check, anything that
> > > > > > overflows the bit field will trigger an EINVAL based on the use of
> > > > > > FAN_INVALID_RESPONSE_MASK in process_access_response().
> > > > > >
> > > 
> > > ...
> > > 
> > > > > >  static ssize_t fanotify_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
> > > > > >  {
> > > > > > -       struct fanotify_response response = { .fd = -1, .response = -1 };
> > > > > > +       struct fanotify_response response;
> > > > > >         struct fsnotify_group *group;
> > > > > >         int ret;
> > > > > > +       size_t size = min(count, sizeof(struct fanotify_response));
> > > > > >
> > > > > >         if (!IS_ENABLED(CONFIG_FANOTIFY_ACCESS_PERMISSIONS))
> > > > > >                 return -EINVAL;
> > > > > >
> > > > > >         group = file->private_data;
> > > > > >
> > > > > > -       if (count < sizeof(response))
> > > > > > +       if (count < offsetof(struct fanotify_response, extra_info_buf))
> > > > > >                 return -EINVAL;
> > > > > 
> > > > > Is this why you decided to shrink the fanotify_response:response field
> > > > > from 32-bits to 16-bits?  I hope not.  I would suggest both keeping
> > > > > the existing response field as 32-bits and explicitly checking for
> > > > > writes that are either the existing/compat length as well as the
> > > > > newer, longer length.
> > > > 
> > > > No.  I shrank it at Jan's suggestion.  I think I agree with you that
> > > > the response field should be kept at u32 as it is defined in userspace
> > > > and purge the doubt about what would happen with a new userspace with
> > > > an old kernel.
> > > 
> > > Hum, for the life of me I cannot find my response you mention here. Can you
> > > send a link so that I can refresh my memory? It has been a long time...
> > 
> > https://listman.redhat.com/archives/linux-audit/2020-October/017066.html
> > https://listman.redhat.com/archives/linux-audit/2020-October/017067.html
> 
> Thanks!
> 
> > > > > > +
> > > > > > +#define FANOTIFY_RESPONSE_EXTRA_LEN_MAX        \
> > > > > > +       (sizeof(union { \
> > > > > > +               struct fanotify_response_audit_rule r; \
> > > > > > +               /* add other extra info structures here */ \
> > > > > > +       }))
> > > > > > +
> > > > > >  struct fanotify_response {
> > > > > >         __s32 fd;
> > > > > > -       __u32 response;
> > > > > > +       __u16 response;
> > > > > > +       __u16 extra_info_type;
> > > > > > +       char extra_info_buf[FANOTIFY_RESPONSE_EXTRA_LEN_MAX];
> > > > > >  };
> > > > > 
> > > > > Since both the kernel and userspace are going to need to agree on the
> > > > > content and formatting of the fanotify_response:extra_info_buf field,
> > > > > why is it hidden behind a char array?  You might as well get rid of
> > > > > that abstraction and put the union directly in the fanotify_response
> > > > > struct.  It is possible you could also get rid of the
> > > > > fanotify_response_audit_rule struct this way too and just access the
> > > > > rule scalar directly.
> > > > 
> > > > This does make sense and my only concern would be a variable-length
> > > > type.  There isn't any reason to hide it.  If userspace chooses to use
> > > > the old interface and omit the type field then it defaults to NONE.
> > > > 
> > > > If future types with variable data are defined, the first field could be
> > > > a u32 that unions with the rule number that won't change the struct
> > > > size.
> > > 
> > > Struct fanotify_response size must not change, it is part of the kernel
> > > ABI. In particular your above change would break userspace code that is
> > > currently working just fine (e.g. allocating 8 bytes and expecting struct
> > > fanotify_response fits there, or just writing sizeof(struct
> > > fanotify_response) as a response while initializing only first 8 bytes).
> > 
> > Many kernel ABIs have been expanded without breaking them.
> > 
> > Is it reasonable for a userspace program to use a kernel structure
> > without also using its size for allocation and initialization?
> 
> Well, I'm not sure whether to call it reasonable but it certainly happens
> and we are generally obliged to keep backwards compatibility (the golden
> "don't break userspace" rule).

There's a lot of stupid things userspace could do that would render it
impossible to ever change or improve anything, including fixing security
bugs, with that absolutist approach.

> > > How I'd suggest doing it now (and I'd like to refresh my memory from my
> > > past emails you mention because in the past I might have thought something
> > > else ;)) is that you add another flag to 'response' field similar to
> > > FAN_AUDIT - like FAN_EXTRA_INFO. If that is present, it means extra info is
> > > to be expected after struct fanotify_response.
> > 
> > That's an interesting possibility...  I'm trying to predict if that
> > would be a problem for an old kernel...  In process_access_response() it
> > would fallthrough to the default case of -EINVAL but do so safely.
> 
> Yes, old kernel would just refuse such response so new userspace can adapt
> to that.

Fair enough.

> > > The extra info would always start with a header like:
> > > 
> > > struct fanotify_response_info_header {
> > >         __u8 info_type;
> > >         __u8 pad;
> > >         __u16 len;		/* This is including the header itself */
> > > }
> > > 
> > > And after such header there would be the 'blob' of data 'len - header size'
> > > long.  We use this same scheme when passing fanotify events to userspace
> > > and it has proven to be lightweight and extensible. It covers the situation
> > > when in the future audit would decide it wants other data (just change data
> > > type), it would also cover the situation when some other subsystem wants
> > > its information passed as well - there can be more structures like this
> > > attached at the end, we can process the response up to the length of the
> > > write.
> > 
> > This reminds me of the RFC2367 (PF_KEYv2, why is that still right there
> > at the tip of my fingers?)
> > 
> > > Now these are just possible future extensions making sure we can extend the
> > > ABI without too much pain. In the current implementation I'd just return
> > > EINVAL whenever more than FANOTIFY_RESPONSE_MAX_LEN (16 bytes) is written 
> > > and do very strict checks on what gets passed in. It is also trivially
> > > backwards compatible (old userspace on new kernel works just fine).
> > 
> > Old userspace on new kernel would work fine with this idea, the v2 patch
> > posted, or with leaving the response field of struct fanotify_response
> > as __u32.
> 
> So I've realized the idea to reduce 'response' field to __u16 will not work
> for big endian architectures (that was a thinko of my old suggestion from
> 2020). Old userspace binaries will break with that.  So we have to keep
> 'response' to __u32 and just add a flag like I'm suggesting now or
> something like that.

Ok, this is the nagging suspicion I had.  This decides it in favour of
not changing that u32 response.

> > > If you want to achieve compatibility of running new userspace on old kernel
> > > (I guess that's desirable), we have group flags for that - like we
> > > introduced FAN_ENABLE_AUDIT to allow for FAN_AUDIT flag in response we now
> > > need to add a flag like FAN_EXTENDED_PERMISSION_INFO telling the kernel it
> > > should expect an allow more info returning for permission events. At the
> > > same time this is the way for userspace to be able to tell whether the
> > > kernel supports this. I know this sounds tedious but that's the cost of
> > > extending the ABI in the compatible way. We've made various API mistakes in
> > > the past having to add weird workarounds to fanotify and we don't want to
> > > repeat those mistakes :).
> > > 
> > > One open question I have is what should the kernel do with 'info_type' in
> > > response it does not understand (in the future when there are possibly more
> > > different info types). It could just skip it because this should be just
> > > additional info for introspection (the only mandatory part is in
> > > fanotify_response, however it could surprise userspace that passed info is
> > > just getting ignored. To solve this we would have to somewhere report
> > > supported info types (maybe in fanotify fdinfo in proc). I guess we'll
> > > cross that bridge when we get to it.
> > 
> > Well, one possibility is to return -EINVAL to signal the kernel is out
> > of date.
> 
> Yes, I think we've settled with Amir on returning -EINVAL in case unknown
> info is spotted.

Good.

> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR

- RGB

--
Richard Guy Briggs <rgb@redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635