Re: [PATCH ghak95] audit: Do not log full CWD path on empty relative paths

[Paul Moore <paul@paul-moore.com>]

On Thu, Aug 2, 2018 at 7:45 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>
> When a relative path has just a single component and we want to emit a
> nametype=PARENT record, the current implementation just reports the full
> CWD path (which is alrady available in the audit context).
>
> This is wrong for three reasons:
> 1. Wasting log space for redundant data (CWD path is already in the CWD
>    record).
> 2. Inconsistency with other PATH records (if a relative PARENT directory
>    path contains at least one component, only the verbatim relative path
>    is logged).
> 3. In some syscalls (e.g. openat(2)) the relative path may not even be
>    relative to the CWD, but to another directory specified as a file
>    descriptor. In that case the logged path is simply plain wrong.
>
> This patch modifies this behavior to simply report "." in the
> aforementioned case, which is equivalent to an "empty" directory path
> and can be concatenated with the actual base directory path (CWD or
> dirfd from openat(2)-like syscall) once support for its logging is added
> later. In the meantime, defaulting to CWD as base directory on relative
> paths (as already done by the userspace tools) will be enough to achieve
> results equivalent to the current behavior.
>
> See: https://github.com/linux-audit/audit-kernel/issues/95
>
> Fixes: 9c937dcc7102 ("[PATCH] log more info for directory entry change events")
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> ---
>  kernel/audit.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/audit.c b/kernel/audit.c
> index 2a8058764aa6..4f18bd48eb4b 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -2127,28 +2127,27 @@ void audit_log_name(struct audit_context *context, struct audit_names *n,
>
>         audit_log_format(ab, "item=%d", record_num);
>
> +       audit_log_format(ab, " name=");
>         if (path)
> -               audit_log_d_path(ab, " name=", path);
> +               audit_log_d_path(ab, NULL, path);
>         else if (n->name) {
>                 switch (n->name_len) {
>                 case AUDIT_NAME_FULL:
>                         /* log the full path */
> -                       audit_log_format(ab, " name=");
>                         audit_log_untrustedstring(ab, n->name->name);
>                         break;
>                 case 0:
>                         /* name was specified as a relative path and the
>                          * directory component is the cwd */
> -                       audit_log_d_path(ab, " name=", &context->pwd);
> +                       audit_log_untrustedstring(ab, ".");

This isn't a comprehensive review, I only gave this a quick look, but
considering you are only logging "." above we can safely use
audit_log_string() and safe a few cycles.

Honestly, looking at the rest of this function, why are we using
audit_log_format() in the case where we are simply writing a string
literal?  While I haven't tested it, simple code inspection would seem
to indicate that audit_log_string() should be much quicker than
audit_log_format()?  I realize this isn't strictly a problem from this
patch, but we probably shouldn't be propagating this mistake any
further.

--
paul moore
www.paul-moore.com