From: David Laight <David.Laight@ACULAB.COM>
To: 'Kees Cook' <keescook@chromium.org>
Cc: 'Christoph Hellwig' <hch@infradead.org>,
"Eric W. Biederman" <ebiederm@xmission.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Al Viro <viro@zeniv.linux.org.uk>,
Luis Chamberlain <mcgrof@kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
"linux-security-module@vger.kernel.org"
<linux-security-module@vger.kernel.org>,
"Serge E. Hallyn" <serge@hallyn.com>,
James Morris <jmorris@namei.org>,
Kentaro Takeda <takedakn@nttdata.co.jp>,
Casey Schaufler <casey@schaufler-ca.com>,
"John Johansen" <john.johansen@canonical.com>
Subject: RE: [PATCH 7/7] exec: Implement kernel_execve
Date: Wed, 15 Jul 2020 16:46:44 +0000 [thread overview]
Message-ID: <dc8371b7e05d4aa49eefcfd402b3fa1e@AcuMS.aculab.com> (raw)
In-Reply-To: <202007150801.27B6690@keescook>
From: Kees Cook <keescook@chromium.org>
> Sent: 15 July 2020 16:09
>
> On Wed, Jul 15, 2020 at 02:55:50PM +0000, David Laight wrote:
> > 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).
>
> *grumpy noises* Yes, but not in practice (if I'm understanding what you
> mean). The expectations of a number of execution environments can be
> really odd-ball. I've tried to collect the notes from over the years in
> prepare_arg_pages()'s comments, and it mostly boils down to "there has
> to be enough room for the exec to start" otherwise the exec ends up in a
> hard-to-debug failure state (i.e. past the "point of no return", where you
> get no useful information about the cause of the SEGV). So the point has
> been to move as many of the setup checks as early as possible and report
> about them if they fail. The argv processing is already very early, but
> it needs to do the limit checks otherwise it'll just break after the exec
> is underway and the process will just SEGV. (And ... some environments
> will attempt to dynamically check the size of the argv space by growing
> until it sees E2BIG, so we can't just remove it and let those hit SEGV.)
Yes - I bet the code is horrid.
I guess the real problem is that you'd need access to the old process's
user addresses and the new process's stack area at the same time.
Unless there is a suitable hole in the old process's address map
any attempted trick will fall foul of cache aliasing on some
architectures - like anything else that does page-loaning.
I'm sure there are hair-brained schemes that might work.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2020-07-15 16:46 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-14 13:27 [PATCH 0/7] Implementing kernel_execve Eric W. Biederman
2020-07-14 13:28 ` [PATCH 1/7] exec: Remove unnecessary spaces from binfmts.h Eric W. Biederman
2020-07-14 21:38 ` Kees Cook
2020-07-15 6:28 ` Christoph Hellwig
2020-07-14 13:29 ` [PATCH 2/7] exec: Factor out alloc_bprm Eric W. Biederman
2020-07-14 21:38 ` Kees Cook
2020-07-15 6:30 ` Christoph Hellwig
2020-07-14 13:29 ` [PATCH 3/7] exec: Move initialization of bprm->filename into alloc_bprm Eric W. Biederman
2020-07-14 21:38 ` Kees Cook
2020-07-15 6:34 ` Christoph Hellwig
2020-07-14 13:30 ` [PATCH 4/7] exec: Move bprm_mm_init " Eric W. Biederman
2020-07-14 21:37 ` Kees Cook
2020-07-15 6:35 ` Christoph Hellwig
2020-07-14 13:30 ` [PATCH 5/7] exec: Factor bprm_execve out of do_execve_common Eric W. Biederman
2020-07-14 21:38 ` Kees Cook
2020-07-15 6:36 ` Christoph Hellwig
2020-07-14 13:31 ` [PATCH 6/7] exec: Factor bprm_stack_limits out of prepare_arg_pages Eric W. Biederman
2020-07-14 21:41 ` Kees Cook
2020-07-15 6:38 ` Christoph Hellwig
2020-07-14 13:31 ` [PATCH 7/7] exec: Implement kernel_execve Eric W. Biederman
2020-07-14 21:49 ` Kees Cook
2020-07-15 6:42 ` Christoph Hellwig
2020-07-15 14:55 ` David Laight
2020-07-15 15:09 ` Kees Cook
2020-07-15 16:46 ` David Laight [this message]
2020-07-15 15:00 ` Kees Cook
2020-07-15 18:20 ` Christoph Hellwig
2020-07-15 6:42 ` Christoph Hellwig
2020-07-15 18:23 ` Eric W. Biederman
2020-07-14 15:32 ` [PATCH 0/7] Implementing kernel_execve Linus Torvalds
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