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Message-ID: <20130719155209.GG4165@ZenIV.linux.org.uk>
Date: Fri, 19 Jul 2013 16:52:09 +0100
From: Al Viro <viro@...IV.linux.org.uk>
To: Patrick McHardy <kaber@...sh.net>
Cc: netdev@...r.kernel.org
Subject: Re: buggy check in netlink_mmap_sendmsg()
On Fri, Jul 19, 2013 at 05:38:46PM +0200, Patrick McHardy wrote:
> On Thu, Jul 18, 2013 at 01:13:58PM +0200, Patrick McHardy wrote:
> > On Sun, Jul 14, 2013 at 10:36:19AM +0100, Al Viro wrote:
> > > This
> > > /* Netlink messages are validated by the receiver before processing.
> > > * In order to avoid userspace changing the contents of the message
> > > * after validation, the socket and the ring may only be used by a
> > > * single process, otherwise we fall back to copying.
> > > */
> > > if (atomic_long_read(&sk->sk_socket->file->f_count) > 2 ||
> > > atomic_read(&nlk->mapped) > 1)
> > > excl = false;
> > > looks very odd. For one thing, descriptor table may be shared, with
> > > one thread calling sendmsg() (which gives f_count equal to 2), while
> > > another calls mmap() just as the first one gets past that check.
> >
> > Another thread calling mmap() should be fine since validation, processing
> > and mmap() all happen under the pg_vec_lock mutex.
Sorry, no. Thread A: calls sendmsg(), gets past the check, loses CPU1 on
e.g. preempt. Thread B on CPU2 calls mmap(2), returns to userland (having
dropped the mutex, of course) and proceeds to play with the mapped area.
Thread A regains CPU and moves on past that if (....), excl still being true.
See the problem?
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