linux-kernel - Re: audit_reusename in getname

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Message-ID: <CAGudoHGqYKTM979s13SAP7fukeAd4NHGTMsxnRWN7A5BpYaCzw@mail.gmail.com>
Date: Thu, 6 Feb 2025 21:34:02 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: Christian Brauner <brauner@...nel.org>, Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	linux-fsdevel <linux-fsdevel@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>, 
	audit@...r.kernel.org, Paul Moore <paul@...l-moore.com>, Eric Paris <eparis@...hat.com>
Subject: Re: audit_reusename in getname_flags

On Thu, Feb 6, 2025 at 9:24 PM Jeff Layton <jlayton@...nel.org> wrote:
>
> On Thu, 2025-02-06 at 20:07 +0100, Mateusz Guzik wrote:
> > You added it in:
> > commit 7ac86265dc8f665cc49d6e60a125e608cd2fca14
> > Author: Jeff Layton <jlayton@...nel.org>
> > Date:   Wed Oct 10 15:25:28 2012 -0400
> >
> >     audit: allow audit code to satisfy getname requests from its names_list
> >
> > Do I read correctly this has no user-visible impact, but merely tries
> > to shave off some memory usage in case of duplicated user bufs?
> >
> > This is partially getting in the way of whacking atomics for filename
> > ref management (but can be worked around).
> >
> > AFAIU this change is not all *that* beneficial in its own right, so
> > should not be a big deal to whack it regardless of what happens with
> > refs? Note it would also remove some branches in the common case as
> > normally audit either has dummy context or there is no match anyway.
>
>
> (cc'ing audit folks and mailing list)
>
> IIRC, having duplicate audit_names records can cause audit to emit
> extra name records in this loop in audit_log_exit():
>
>         list_for_each_entry(n, &context->names_list, list) {
>                 if (n->hidden)
>                         continue;
>                 audit_log_name(context, n, NULL, i++, &call_panic);
>         }
>
>
> ...which is something you probably want to avoid.

Well in this case I would argue the current code is buggy, unless I'm
misunderstanding something.

audit_log_name in particular logs:
  1550 │       if (n->ino != AUDIT_INO_UNSET)
  1551 │               audit_log_format(ab, " inode=%lu dev=%02x:%02x
mode=%#ho ouid=%u ogid=%u rdev=%02x
  1552 │                                n->ino,
  1553 │                                MAJOR(n->dev),
  1554 │                                MINOR(n->dev),
  1555 │                                n->mode,
  1556 │                                from_kuid(&init_user_ns, n->uid),
  1557 │                                from_kgid(&init_user_ns, n->gid),
  1558 │                                MAJOR(n->rdev),
  1559 │                                MINOR(n->rdev));

As in it grabs the properties of the found inode.

Suppose the 2 lookups of the same path name found 2 different inodes
as someone was mucking with the filesystem at the same time.

Then this is going to *fail* to record the next inode.

So if any dedup is necessary, it should be done by audit when logging imo.

-- 
Mateusz Guzik <mjguzik gmail.com>