linux-kernel - RE: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes

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Date:   Mon, 15 Jun 2020 08:27:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christian Brauner' <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>
CC:     Sargun Dhillon <sargun@...gun.me>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Robert Sesek <rsesek@...gle.com>,
        Chris Palmer <palmer@...gle.com>, Jann Horn <jannh@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "containers@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Matt Denton <mpdenton@...gle.com>, Tejun Heo <tj@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: RE: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to
 move fds across processes

From: Christian Brauner
> Sent: 12 June 2020 19:28
...
> > > 	if (size < 32)
> > > 		return -EINVAL;
> > > 	if (size > PAGE_SIZE)
> > > 		return -E2BIG;
> >
> > (Tanget: what was the reason for copy_struct_from_user() not including
> > the min/max check? I have a memory of Al objecting to having an
> > "internal" limit?)
> 
> Al didn't want the PAGE_SIZE limit in there because there's nothing
> inherently wrong with copying insane amounts of memory.

The problem is really allowing a user process to allocate
unbounded blocks of memory, not the copy itself.

The limit for IOW() etc is 16k - not a problem.
If a 32bit size is set to just under 4GB so you really want
to allocate 4GB of memory then find the request is garbage.
Seems like a nice DoS attack.
A 64bit size can be worse.

Potentially the limit should be in memdup_user() itself.
And possibly an extra parameter giving a per-call lower? limit.

	David

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