[PATCH v6 0/3] fanotify: Allow user space to pass back additional audit info

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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 to audit the triggering 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.  This
is done by providing a a fully-formed FAN_INFO extension but setting the
fd to FAN_NOFD.  On a capable kernel, it will succeed but issue no audit
record, whereas on an older kernel it will fail.

The audit function was updated to log the additional information in the
AUDIT_FANOTIFY record. The following are examples of the new record
  type=FANOTIFY msg=audit(1600385147.372:590): resp=2 fan_type=1 fan_info=3137 subj_trust=3 obj_trust=5
  type=FANOTIFY msg=audit(1659730979.839:284): resp=1 fan_type=0 fan_info=3F subj_trust=? obj_trust=?

- first version by Steve Grubb <sgrubb@xxxxxxxxxx>
Link: https://lore.kernel.org/r/2042449.irdbgypaU6@x2

- 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

- 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
- switch to u32 for internal and __u32 for uapi
Link: https://lore.kernel.org/all/cover.1652730821.git.rgb@xxxxxxxxxx

- 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/all/cover.1659996830.git.rgb@xxxxxxxxxx

- fixed warnings in p2/4 and p3/4 found by <lkp@xxxxxxxxx>
- restore original behaviour for !FAN_INFO case and fanotify_get_response()
- rename member audit_rule to rule_number
- eliminate memory leak of info_buf on failure (no longer dynamic)
- rename buf:info, count:info_len, c:remain, ib:infop
- fix pr_debug
- return -ENOENT on FAN_INFO and fd==FAN_NOFD to signal new kernel
- fanotify_write() remove redundant size check
- add u32 subj_trust obj_trust fields with unknown value "2"
- split out to helper process_access_response_info()
- restore finish_permission_event() response_struct to u32
- assume and enforce one rule to audit, pass struct directly to __audit_fanotify()
- change fanotify_perm_event struct to union hdr/audir_rule
- add vspace to fanotify_write() and process_access_response_info()
- squash 3/4 with 4/4
- fix v3 and v4 links
Link: https://lore.kernel.org/all/cover.1670606054.git.rgb@xxxxxxxxxx

- simplify __audit_fanotify() from audit_log_format/audit_log_n_hex to audit_log/%X
- add comment to clarify {subj,obj}_trust values
- remove fd processing from process_access_response_info()
- return info_len immediately from process_access_response() on FAN_NOFD after process_access_response_info()
Link: https://lore.kernel.org/all/cover.1673989212.git.rgb@xxxxxxxxxx

Richard Guy Briggs (3):
  fanotify: Ensure consistent variable type for response
  fanotify: define struct members to hold response decision context
  fanotify,audit: Allow audit to use the full permission event response

 fs/notify/fanotify/fanotify.c      |  8 ++-
 fs/notify/fanotify/fanotify.h      |  6 +-
 fs/notify/fanotify/fanotify_user.c | 88 ++++++++++++++++++++++--------
 include/linux/audit.h              |  9 +--
 include/linux/fanotify.h           |  5 ++
 include/uapi/linux/fanotify.h      | 30 +++++++++-
 kernel/auditsc.c                   | 16 +++++-
 7 files changed, 129 insertions(+), 33 deletions(-)


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