netdev - Re: [syzbot] [mptcp?] general protection fault in proc

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Message-ID: <20250105195434.GJ1977892@ZenIV>
Date: Sun, 5 Jan 2025 19:54:34 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Matthieu Baerts <matttbe@...nel.org>, davem@...emloft.net,
	geliang@...nel.org, horms@...nel.org, kuba@...nel.org,
	linux-kernel@...r.kernel.org, martineau@...nel.org,
	mptcp@...ts.linux.dev, netdev@...r.kernel.org, pabeni@...hat.com,
	syzkaller-bugs@...glegroups.com,
	syzbot <syzbot+e364f774c6f57f2c86d1@...kaller.appspotmail.com>
Subject: Re: [syzbot] [mptcp?] general protection fault in proc_scheduler

On Sun, Jan 05, 2025 at 05:52:19PM +0100, Eric Dumazet wrote:
> On Sun, Jan 5, 2025 at 12:29 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Sun, Jan 05, 2025 at 09:32:36AM +0100, Eric Dumazet wrote:
> >
> > > According to grep, we have many other places directly reading
> > > current->nsproxy->net_ns
> > > For instance in net/sctp/sysctl.c
> > > Should we change them all ?
> >
> > Depends - do you want their contents match the netns of opener (as,
> > AFAICS, for ipv4 sysctls) or that of the reader?
> 
> I am only worried that a malicious user could crash the host with
> current kernels,
> not about this MPTP crash, but all unaware users of current->nsproxy
> in sysctl handlers.

I don't hate your mitigation in proc_sysctl.c, but IMO there are two
problems mixed here - one is that we probably should have access
to per-netns sysctl table act on the netns it had been created for,
which may not coincide with reader's/writer's netns and another is that
access to current->nsproxy->netns would simply oops if attempted when
current->nsproxy had been dropped.

So I suspect that current->nsproxy->netns shouldn't be used in
per-netns sysctls for consistency sake (note that it can get more
serious than just consistency, if you have e.g. a spinlock taken
in something hanging off current netns to protect access to
something table->data points to).

As for the mitigation in fs/proc/proc_sysctl.c... might be useful,
if it comes with a clear comment about the reasons it's there.