Re: [PATCH v2 2/3] fanotify: define struct members to hold response decision context

From: Paul Moore
Date: Wed May 04 2022 - 23:33:17 EST

On Tue, May 3, 2022 at 9:33 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> On 2022-05-02 20:16, Paul Moore wrote:
> > On Thu, Apr 28, 2022 at 8:45 PM Richard Guy Briggs <rgb@xxxxxxxxxx> 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().
> > >
> > > Suggested-by: Steve Grubb <sgrubb@xxxxxxxxxx>
> > > Link: https://lore.kernel.org/r/2745105.e9J7NaK4W3@x2
> > > Suggested-by: Jan Kara <jack@xxxxxxx>
> > > Link: https://lore.kernel.org/r/20201001101219.GE17860@xxxxxxxxxxxxxx
> > > Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> > > Link: https://lore.kernel.org/r/17660b3f2817e5c0a19d1e9e5d40b53ff4561845.1651174324.git.rgb@xxxxxxxxxx
> > > ---
> > > fs/notify/fanotify/fanotify.c | 1 -
> > > fs/notify/fanotify/fanotify.h | 4 +-
> > > fs/notify/fanotify/fanotify_user.c | 59 ++++++++++++++++++++----------
> > > include/linux/fanotify.h | 3 ++
> > > include/uapi/linux/fanotify.h | 27 +++++++++++++-
> > > 5 files changed, 72 insertions(+), 22 deletions(-)

...

> > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> > > index 694516470660..f1ff4cf683fb 100644
> > > --- a/fs/notify/fanotify/fanotify_user.c
> > > +++ b/fs/notify/fanotify/fanotify_user.c
> > > @@ -289,13 +289,19 @@ static int create_fd(struct fsnotify_group *group, struct path *path,
> > > */
> > > static void finish_permission_event(struct fsnotify_group *group,
> > > struct fanotify_perm_event *event,
> > > - __u32 response)
> > > + struct fanotify_response *response)
> > > __releases(&group->notification_lock)
> > > {
> > > bool destroy = false;
> > >
> > > assert_spin_locked(&group->notification_lock);
> > > - event->response = response;
> > > + event->response = response->response;
> > > + event->extra_info_type = response->extra_info_type;
> > > + switch (event->extra_info_type) {
> > > + case FAN_RESPONSE_INFO_AUDIT_RULE:
> > > + memcpy(event->extra_info_buf, response->extra_info_buf,
> > > + sizeof(struct fanotify_response_audit_rule));
> >
> > Since the fanotify_perm_event:extra_info_buf and
> > fanotify_response:extra_info_buf are the same type/length, and they
> > will be the same regardless of the extra_info_type field, why not
> > simply get rid of the above switch statement and do something like
> > this:
> >
> > memcpy(event->extra_info_buf, response->extra_info_buf,
> > sizeof(response->extra_info_buf));
>
> I've been wrestling with the possibility of doing a split between what
> is presented to userspace and what's used in the kernel for struct
> fanotify_response, while attempting to future-proof it.

You really only need to worry about what is presented to userspace,
the kernel internals can always change if needed. Right now I would
focus on making sure the userspace visible data structures are done
properly: preserve the existing data offsets/lengths, and ensure that
the new additions do not make it harder to extend the structure again
in the future.

> > > @@ -827,26 +845,25 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
> > >
> > > 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.

I'm struggling to think of why shrinking an existing field is a good
idea. Unfortunately, there is a possibility that any problems this
would cause might not be caught until it has been in a couple of
kernel releases and some applications have been written/updated to use
the new struct definition, at which point restoring the field to a u32
value will break all of these new applications.

I think changing the fanotify_response:response field is an
unnecessary risk, and I'll leave it at that.

> > > diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
> > > index e8ac38cc2fd6..efb5a3a6f814 100644
> > > --- a/include/uapi/linux/fanotify.h
> > > +++ b/include/uapi/linux/fanotify.h
> > > @@ -179,9 +179,34 @@ struct fanotify_event_info_error {
> > > __u32 error_count;
> > > };
> > >
> > > +/*
> > > + * User space may need to record additional information about its decision.
> > > + * The extra information type records what kind of information is included.
> > > + * The default is none. We also define an extra informaion buffer whose
> > > + * size is determined by the extra information type.
> > > + *
> > > + * If the context type is Rule, then the context following is the rule number
> > > + * that triggered the user space decision.
> > > + */
> > > +
> > > +#define FAN_RESPONSE_INFO_AUDIT_NONE 0
> > > +#define FAN_RESPONSE_INFO_AUDIT_RULE 1
> > > +
> > > +struct fanotify_response_audit_rule {
> > > + __u32 rule;
> > > +};
> > > +
> > > +#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.

There is no reason you couldn't put flexible-array field in a union if
that is what was needed. Of you could have the flexible-array field
outside of the union and use a union field as the length value.
There's probably other clever solutions to this too.

--
paul-moore.com