net/unix: drop obsolete fd-recursion limits

All unix sockets now account inflight FDs to the respective sender.
This was introduced in:

    commit 712f4aad406bb1ed67f3f98d04c044191f0ff593
    Author: willy tarreau <w@1wt.eu>
    Date:   Sun Jan 10 07:54:56 2016 +0100

        unix: properly account for FDs passed over unix sockets

and further refined in:

    commit 415e3d3e90ce9e18727e8843ae343eda5a58fad6
    Author: Hannes Frederic Sowa <hannes@stressinduktion.org>
    Date:   Wed Feb 3 02:11:03 2016 +0100

        unix: correctly track in-flight fds in sending process user_struct

Hence, regardless of the stacking depth of FDs, the total number of
inflight FDs is limited, and accounted. There is no known way for a
local user to exceed those limits or exploit the accounting.

Furthermore, the GC logic is independent of the recursion/stacking depth
as well. It solely depends on the total number of inflight FDs,
regardless of their layout.

Lastly, the current `recursion_level' suffers a TOCTOU race, since it
checks and inherits depths only at queue time. If we consider `A<-B' to
mean `queue-B-on-A', the following sequence circumvents the recursion
level easily:

    A<-B
       B<-C
          C<-D
             ...
               Y<-Z

resulting in:

    A<-B<-C<-...<-Z

With all of this in mind, lets drop the recursion limit. It has no
additional security value, anymore. On the contrary, it randomly
confuses message brokers that try to forward file-descriptors, since
any sendmsg(2) call can fail spuriously with ETOOMANYREFS if a client
maliciously modifies the FD while inflight.

Cc: Alban Crequy <alban.crequy@collabora.co.uk>
Cc: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 0 insertions, 1 deletions

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 678e4d6fa317..3b3194b2fc65 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -58,7 +58,6 @@ struct unix_sock {
 	struct list_head	link;
 	atomic_long_t		inflight;
 	spinlock_t		lock;
-	unsigned char		recursion_level;
 	unsigned long		gc_flags;
 #define UNIX_GC_CANDIDATE	0
 #define UNIX_GC_MAYBE_CYCLE	1