linux-kernel - RE: [PATCH 7/7] exec: Implement kernel

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Date:   Wed, 15 Jul 2020 14:55:50 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@...radead.org>,
        Kees Cook <keescook@...omium.org>
CC:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Luis Chamberlain <mcgrof@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "John Johansen" <john.johansen@...onical.com>
Subject: RE: [PATCH 7/7] exec: Implement kernel_execve

From: Christoph Hellwig
> Sent: 15 July 2020 07:43
> Subject: Re: [PATCH 7/7] exec: Implement kernel_execve
> 
> On Tue, Jul 14, 2020 at 02:49:23PM -0700, Kees Cook wrote:
> > On Tue, Jul 14, 2020 at 08:31:40AM -0500, Eric W. Biederman wrote:
> > > +static int count_strings_kernel(const char *const *argv)
> > > +{
> > > +	int i;
> > > +
> > > +	if (!argv)
> > > +		return 0;
> > > +
> > > +	for (i = 0; argv[i]; ++i) {
> > > +		if (i >= MAX_ARG_STRINGS)
> > > +			return -E2BIG;
> > > +		if (fatal_signal_pending(current))
> > > +			return -ERESTARTNOHAND;
> > > +		cond_resched();
> > > +	}
> > > +	return i;
> > > +}
> >
> > I notice count() is only ever called with MAX_ARG_STRINGS. Perhaps
> > refactor that too? (And maybe rename it to count_strings_user()?)

Thinks....
If you setup env[] and argv[] on the new user stack early in exec processing
then you may not need any limits at all - except the size of the user stack.
Even the get_user() loop will hit an invalid address before the counter
wraps (provided it is unsigned long).

	David

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