linux-kernel - Userland used in writeback path was Re: [PATCH] nbd: restrict sockets to TCP and UDP

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Message-ID: <aR2JjR1yKHCCPalO@duo.ucw.cz>
Date: Wed, 19 Nov 2025 10:10:37 +0100
From: Pavel Machek <pavel@....cz>
To: "Richard W.M. Jones" <rjones@...hat.com>, akpm@...ux-foundation.org,
	david@...nel.org, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, rppt@...nel.org, vbabka@...e.cz,
	surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org
Cc: Eric Dumazet <edumazet@...gle.com>, Josef Bacik <josef@...icpanda.com>,
	Jens Axboe <axboe@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	syzbot+e1cd6bd8493060bd701d@...kaller.appspotmail.com,
	Mike Christie <mchristi@...hat.com>,
	Yu Kuai <yukuai1@...weicloud.com>, linux-block@...r.kernel.org,
	nbd@...er.debian.org
Subject: Userland used in writeback path was Re: [PATCH] nbd: restrict
 sockets to TCP and UDP

On Tue 2025-11-18 18:16:23, Richard W.M. Jones wrote:
> On Tue, Nov 18, 2025 at 06:56:33PM +0100, Pavel Machek wrote:
> > Hi!
> > 
> > > Recently, syzbot started to abuse NBD with all kinds of sockets.
> > > 
> > > Commit cf1b2326b734 ("nbd: verify socket is supported during setup")
> > > made sure the socket supported a shutdown() method.
> > > 
> > > Explicitely accept TCP and UNIX stream sockets.
> > 
> > Note that running nbd server and client on same machine is not safe in
> > read-write mode. It may deadlock under low memory conditions.
> > 
> > Thus I'm not sure if we should accept UNIX sockets.
> 
> Both nbd-client and nbdkit have modes where they can mlock themselves
> into RAM.

kernel needs memory. It issues write-back to get some.
nbd-client does syscall. Maybe writing to storage?
That syscall does kmalloc().
That kmalloc now needs something like PF_MEMALLOC flag.

mlock() is not enough.

Best regards,
							Pavel
-- 
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