Re: [PATCH Astable 5.15/6.1/6.6] af_unix: Clear oob_skb in scan_inflight().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Lee Jones <lee@kernel.org>
Date: Fri, 16 May 2025 09:18:43 +0100
> On Wed, 05 Mar 2025, Kuniyuki Iwashima wrote:
> 
> > +Paolo
> > 
> > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Date: Wed, 5 Mar 2025 15:08:26 +0100
> > > On Mon, Mar 03, 2025 at 07:01:49PM -0800, Kuniyuki Iwashima wrote:
> > > > Embryo socket is not queued in gc_candidates, so we can't drop
> > > > a reference held by its oob_skb.
> > > > 
> > > > Let's say we create listener and embryo sockets, send the
> > > > listener's fd to the embryo as OOB data, and close() them
> > > > without recv()ing the OOB data.
> > > > 
> > > > There is a self-reference cycle like
> > > > 
> > > >   listener -> embryo.oob_skb -> listener
> > > > 
> > > > , so this must be cleaned up by GC.  Otherwise, the listener's
> > > > refcnt is not released and sockets are leaked:
> > > > 
> > > >   # unshare -n
> > > >   # cat /proc/net/protocols | grep UNIX-STREAM
> > > >   UNIX-STREAM 1024      0      -1   NI       0   yes  kernel ...
> > > > 
> > > >   # python3
> > > >   >>> from array import array
> > > >   >>> from socket import *
> > > >   >>>
> > > >   >>> s = socket(AF_UNIX, SOCK_STREAM)
> > > >   >>> s.bind('\0test\0')
> > > >   >>> s.listen()
> > > >   >>>
> > > >   >>> c = socket(AF_UNIX, SOCK_STREAM)
> > > >   >>> c.connect(s.getsockname())
> > > >   >>> c.sendmsg([b'x'], [(SOL_SOCKET, SCM_RIGHTS, array('i', [s.fileno()]))], MSG_OOB)
> > > >   1
> > > >   >>> quit()
> > > > 
> > > >   # cat /proc/net/protocols | grep UNIX-STREAM
> > > >   UNIX-STREAM 1024      3      -1   NI       0   yes  kernel ...
> > > >                         ^^^
> > > >                         3 sockets still in use after FDs are close()d
> > > > 
> > > > Let's drop the embryo socket's oob_skb ref in scan_inflight().
> > > > 
> > > > This also fixes a racy access to oob_skb that commit 9841991a446c
> > > > ("af_unix: Update unix_sk(sk)->oob_skb under sk_receive_queue
> > > > lock.") fixed for the new Tarjan's algo-based GC.
> > > > 
> > > > Fixes: 314001f0bf92 ("af_unix: Add OOB support")
> > > > Reported-by: Lei Lu <llfamsec@gmail.com>
> > > > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > > > ---
> > > > This has no upstream commit because I replaced the entire GC in
> > > > 6.10 and the new GC does not have this bug, and this fix is only
> > > > applicable to the old GC (<= 6.9), thus for 5.15/6.1/6.6.
> > > 
> > > You need to get the networking maintainers to review and agree that this
> > > is ok for us to take, as we really don't want to take "custom" stuff
> > > like thi s at all.
> > 
> > Paolo, could you take a look at this patch ?
> > https://lore.kernel.org/netdev/20250304030149.82265-1-kuniyu@amazon.com/
> > 
> > 
> > > Why not just take the commits that are in newer
> > > kernels instead?
> > 
> > That will be about 20 patches that rewrite the most lines of
> > net/unix/garbage.c and cannot be applied cleanly.
> > 
> > I think backporting these commits is overkill to fix a small
> > bug that can be fixed with a much smaller diff.
> > 
> > 927fa5b3e4f5 af_unix: Fix uninit-value in __unix_walk_scc()
> > 041933a1ec7b af_unix: Fix garbage collection of embryos carrying OOB with SCM_RIGHTS
> > 7172dc93d621 af_unix: Add dead flag to struct scm_fp_list.
> > 1af2dface5d2 af_unix: Don't access successor in unix_del_edges() during GC.
> > fd86344823b5 af_unix: Try not to hold unix_gc_lock during accept().
> > 118f457da9ed af_unix: Remove lock dance in unix_peek_fds().
> > 4090fa373f0e af_unix: Replace garbage collection algorithm.
> > a15702d8b3aa af_unix: Detect dead SCC.
> > bfdb01283ee8 af_unix: Assign a unique index to SCC.
> > ad081928a8b0 af_unix: Avoid Tarjan's algorithm if unnecessary.
> > 77e5593aebba af_unix: Skip GC if no cycle exists.
> > ba31b4a4e101 af_unix: Save O(n) setup of Tarjan's algo.
> > dcf70df2048d af_unix: Fix up unix_edge.successor for embryo socket.
> > 3484f063172d af_unix: Detect Strongly Connected Components.
> > 6ba76fd2848e af_unix: Iterate all vertices by DFS.
> > 22c3c0c52d32 af_unix: Bulk update unix_tot_inflight/unix_inflight when queuing skb.
> > 42f298c06b30 af_unix: Link struct unix_edge when queuing skb.
> > 29b64e354029 af_unix: Allocate struct unix_edge for each inflight AF_UNIX fd.
> > 1fbfdfaa5902 af_unix: Allocate struct unix_vertex for each inflight AF_UNIX fd.
> 
> Okay, I've taken some time to apply a set to linux-6.6.y that looks
> similar to this [0].  Would someone please give me some advice on how to
> test it please?  Are there some unit tests I could run to ensure that
> everything is working as expected?

Thanks for working on this! and sorry for the late response, I'm
personally busy this month.

Could you run tools/testing/selftests/net/af_unix/scm_rights.c
with KASAN and kmemleak ?

The test is added by

2a79651bf2fa selftest: af_unix: Add test case for backtrack after finalising SCC.
e060e433e512 selftest: af_unix: Make SCM_RIGHTS into OOB data.
2aa0cff26ed5 selftest: af_unix: Test GC for SCM_RIGHTS.

The 2nd commit will cover the test case of the python script in
the patch of this thread.