Re: [PATCH v4 0/4] fanotify: Allow user space to pass back additional audit info

[Amir Goldstein]

[+linux-api]

On Tue, Aug 9, 2022 at 7:23 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>
> The Fanotify API can be used for access control by requesting permission
> event notification. The user space tooling that uses it may have a
> complicated policy that inherently contains additional context for the
> decision. If this information were available in the audit trail, policy
> writers can close the loop on debugging policy. Also, if this additional
> information were available, it would enable the creation of tools that
> can suggest changes to the policy similar to how audit2allow can help
> refine labeled security.
>
> This patchset defines a new flag (FAN_INFO) and new extensions that
> define additional information which are appended after the response
> structure returned from user space on a permission event.  The appended
> information is organized with headers containing a type and size that
> can be delegated to interested subsystems.  One new information type is
> defined for audit rule number.
>
> A newer kernel will work with an older userspace and an older kernel
> will behave as expected and reject a newer userspace, leaving it up to
> the newer userspace to test appropriately and adapt as necessary.

Since you did not accept my suggestion of FAN_TEST response code [1],
I am not really sure how you envision that "adapt as necessary" part.

A well designed UAPI should allow newer userspace to check for kernel
support of FAN_INFO on initialization.

For example, without this property of UAPI, it is going to be hard to write an
LTP test for the new functionality that does not run on older kernels.

As far as I can tell, your proposed UAPI does not provide this functionality.
It allows newer userspace to check for support of FAN_INFO only as a
response to a permission event.

You never replied to my question to explain why you think FAN_TEST
complicated things. The only purpose of FAN_TEST is for userspace
to be able to test FAN_INFO kernel support without providing a valid fd.

[1] https://lore.kernel.org/linux-fsdevel/CAOQ4uxi+8HUqyGxQBNMqSong92nreOWLKdy9MCrYg8wgW9Dj4g@xxxxxxxxxxxxxx/

>
> The audit function was updated to log the additional information in the
> AUDIT_FANOTIFY record. The following is an example of the new record
> format:
>
> type=FANOTIFY msg=audit(1600385147.372:590): resp=2 fan_type=1 fan_info=3F
>
> changelog:
> v1:
> - first version by Steve Grubb <sgrubb@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/2042449.irdbgypaU6@x2
>
> v2:
> - enhancements suggested by Jan Kara <jack@xxxxxxx>
> - 1/3 change %d to %u in pr_debug
> - 2/3 change response from __u32 to __u16
> - mod struct fanotify_response and fanotify_perm_event add extra_info_type, extra_info_buf
> - extra_info_buf size max FANOTIFY_MAX_RESPONSE_EXTRA_LEN, add struct fanotify_response_audit_rule
> - extend debug statements
> - remove unneeded macros
> - [internal] change interface to finish_permission_event() and process_access_response()
> - 3/3 update format of extra information
> - [internal] change interface to audit_fanotify()
> - change ctx_type= to fan_type=
> Link: https://lore.kernel.org/r/cover.1651174324.git.rgb@xxxxxxxxxx
>
> v3:
> - 1/3 switch {,__}audit_fanotify() from uint to u32
> - 2/3 re-add fanotify_get_response switch case FAN_DENY: to avoid unnecessary churn
> - add FAN_EXTRA flag to indicate more info and break with old kernel
> - change response from u16 to u32 to avoid endian issues
> - change extra_info_buf to union
> - move low-cost fd check earlier
> - change FAN_RESPONSE_INFO_AUDIT_NONE to FAN_RESPONSE_INFO_NONE
> - switch to u32 for internal and __u32 for uapi
> Link: https://lore.kernel.org/r/cover.1652724390.git.rgb@xxxxxxxxxx

Link seems broken?

>
> v4:
> - scrap FAN_INVALID_RESPONSE_MASK in favour of original to catch invalid response == 0
> - introduce FANOTIFY_RESPONSE_* macros
> - uapi: remove union
> - keep original struct fanotify_response, add fan_info infra starting with audit reason
> - uapi add struct fanotify_response_info_header{type/pad/len} and struct fanotify_response_info_audit_rule{hdr/rule}
> - rename fan_ctx= to fan_info=, FAN_EXTRA to FAN_INFO
> - change event struct from type/buf to len/buf
> - enable multiple info extensions in one message
> - hex encode fan_info in __audit_fanotify()
> - record type FANOTIFY extended to "type=FANOTIFY msg=audit(1659730979.839:284): resp=1 fan_type=0 fan_info=3F"
> Link: https://lore.kernel.org/r/cover.1659981772.git.rgb@xxxxxxxxxx

Link seems broken?

Thanks,
Amir.