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

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Message-ID: <20151229203527.GA13826@1wt.eu>
Date:	Tue, 29 Dec 2015 21:35:27 +0100
From:	Willy Tarreau <w@....eu>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Dumazet <edumazet@...gle.com>, socketpair@...il.com
Subject: Re: [PATCH] unix: properly account for FDs passed over unix sockets

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@...il.com
> >Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> >Signed-off-by: Willy Tarreau <w@....eu>
> 
> 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 :-/

Thanks,
Willy

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