[PATCH v3 1/1] audit: Record fanotify access control decisions

[Amir Goldstein]

On Mon, Sep 25, 2017 at 6:19 PM, Steve Grubb <sgrubb at redhat.com> wrote:
> Hello,
>
> The fanotify interface allows user space daemons to make access
> control decisions. Under common criteria requirements, we need to
> optionally record decisions based on policy. This patch adds a bit mask,
> FAN_AUDIT, that a user space daemon can 'or' into the response decision
> which will tell the kernel that it made a decision and record it.
>
> It would be used something like this in user space code:
>
>   response.response = FAN_DENY | FAN_AUDIT;
>   write(fd, &response, sizeof(struct fanotify_response));
>
> When the syscall ends, the audit system will record the decision as a
> AUDIT_FANOTIFY auxiliary record to denote that the reason this event
> occurred is the result of an access control decision from fanotify
> rather than DAC or MAC policy.
>
> A sample event looks like this:
>
> type=PATH msg=audit(1504310584.332:290): item=0 name="./evil-ls"
> inode=1319561 dev=fc:03 mode=0100755 ouid=1000 ogid=1000 rdev=00:00
> obj=unconfined_u:object_r:user_home_t:s0 nametype=NORMAL
> type=CWD msg=audit(1504310584.332:290): cwd="/home/sgrubb"
> type=SYSCALL msg=audit(1504310584.332:290): arch=c000003e syscall=2
> success=no exit=-1 a0=32cb3fca90 a1=0 a2=43 a3=8 items=1 ppid=901
> pid=959 auid=1000 uid=1000 gid=1000 euid=1000 suid=1000
> fsuid=1000 egid=1000 sgid=1000 fsgid=1000 tty=pts1 ses=3 comm="bash"
> exe="/usr/bin/bash" subj=unconfined_u:unconfined_r:unconfined_t:
> s0-s0:c0.c1023 key=(null)
> type=FANOTIFY msg=audit(1504310584.332:290): resp=2
>
> Prior to using the audit flag, the developer needs to call
> fanotify_init or'ing in FAN_AUDIT_ENABLE to ensure that the kernel
> supports auditing. The calling process must also have the CAP_AUDIT_WRITE
> capability.
>
> Signed-off-by: sgrubb <sgrubb at redhat.com>

Sorry, found more nits...

> @@ -805,6 +808,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
>                 group->fanotify_data.max_marks = FANOTIFY_DEFAULT_MAX_MARKS;
>         }
>
> +       if (flags & FAN_ENABLE_AUDIT) {
> +               fd = -EPERM;
> +               if (!capable(CAP_AUDIT_WRITE))
> +                       goto out_destroy_group;
> +               group->audit_enabled = 1;
> +       }
> +

App should not be able to enable audit if audit is not configured in kernel.
So there should be code that returns EINVAL for flag FAN_ENABLE_AUDIT
if CONFIG_AUDITSYSCALL is not defined.
Same deal as flag FAN_ALL_PERM_EVENTS and kernel config
CONFIG_FANOTIFY_ACCESS_PERMISSIONS in fanotify_mark().
Perhaps it is better not to extend FAN_ALL_INIT_FLAGS and explicitly
test (flags & ~(FAN_ALL_INIT_FLAGS | FAN_ENABLE_AUDIT)) in fanotify_init()
under ifdef, as done in fanotify_mark()?

> diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
> index c6c6931..8f7ea68 100644
> --- a/include/linux/fsnotify_backend.h
> +++ b/include/linux/fsnotify_backend.h
> @@ -193,6 +193,7 @@ struct fsnotify_group {
>                 } fanotify_data;
>  #endif /* CONFIG_FANOTIFY */
>         };
> +       unsigned int audit_enabled;

This one should be bool and it belongs inside fanotify_data union member.
Also, you need to translate it back to FAN_ENABLE_AUDIT in
fanotify_show_fdinfo().

Amir.