Re: [PATCH] unix: properly account for FDs passed over unix sockets

[Hannes Frederic Sowa]

On 29.12.2015 21:35, Willy Tarreau wrote:
> On Tue, Dec 29, 2015 at 03:48:45PM +0100, Hannes Frederic Sowa wrote:
> > On 28.12.2015 15:14, Willy Tarreau wrote:
> > > It is possible for a process to allocate and accumulate far more FDs than
> > > the process' limit by sending them over a unix socket then closing them
> > > to keep the process' fd count low.
> > >
> > > This change addresses this problem by keeping track of the number of FDs
> > > in flight per user and preventing non-privileged processes from having
> > > more FDs in flight than their configured FD limit.
> > >
> > > Reported-by: socketpair@xxxxxxxxx
> > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Willy Tarreau <w@xxxxxx>
> >
> > Thanks for the patch!
> >
> > I think this does not close the DoS attack completely as we duplicate
> > fds if the reader uses MSG_PEEK on the unix domain socket and thus
> > clones the fd. Have I overlooked something?
>
> I didn't know this behaviour. However, then the fd remains in flight, right ?
> So as long as it's not removed from the queue, the sender cannot add more
> than its FD limit. I may be missing something obvious though :-/

Yes, it remains in flight.

The MSG_PEEK code should not be harmful and the patch is good as is. I 
first understood from the published private thread, that it is possible 
for a program to exceed the rlimit of fds. But the DoS is only by 
keeping the fds in flight and not attaching them to any program.

__alloc_fd, called on the receiver side, does check for the rlimit 
maximum anyway, so I don't see a loophole anymore:

Acked-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>

Another idea would be to add the amount of memory used to manage the fds 
to sock_rmem/wmem but I don't see any advantages or disadvantages.

Thanks!

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/