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

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Mar 05, 2025 at 10:10:41AM -0800, 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.

Sure, but now all fixes made upstream after these changes will not apply
to older kernels at all, making supporting this old one-off change
harder and harder over time.

But I'll defer to the maintainers here as to what they want.  Taking 20+
patches in a stable tree is trivial for us, not a problem at all.

thanks,

greg k-h