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Date: Sat, 4 Nov 2017 00:44:51 +0000
From: Al Viro <viro@...IV.linux.org.uk>
To: Kees Cook <keescook@...omium.org>
Cc: Laura Abbott <labbott@...hat.com>, kernel-hardening@...ts.openwall.com,
LKML <linux-kernel@...r.kernel.org>,
Mark Rutland <mark.rutland@....com>, X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH 2/2] x86: Allow paranoid __{get,put}_user
On Sat, Nov 04, 2017 at 12:24:30AM +0000, Al Viro wrote:
> On Fri, Nov 03, 2017 at 05:14:05PM -0700, Kees Cook wrote:
> > > x86 turns out to be easier since the safe and unsafe paths are mostly
> > > disjoint so we don't have to worry about gcc optimizing out access_ok.
> > > I tweaked the Kconfig to someting a bit more generic.
> > >
> > > The size increase was ~8K in text with a config I tested.
> >
> > Specifically, this feature would have caught the waitid() bug in 4.13
> > immediately.
>
> You mean, as soon as waitid() was given a kernel address. At which point
> you'd get a shiny way to generate a BUG(), and if something like that
> happened under a mutex - it's even more fun...
>
> > > +config PARANOID_UACCESS
> > > + bool "Use paranoid uaccess primitives"
> > > + depends on ARCH_HAS_PARANOID_UACCESS
> > > + help
> > > + Forces access_ok() checks in __get_user(), __put_user(), and other
> > > + low-level uaccess primitives which usually do not have checks. This
> > > + can limit the effect of missing access_ok() checks in higher-level
> > > + primitives, with a runtime performance overhead in some cases and a
> > > + small code size overhead.
>
> IMO that's the wrong way to go - what we need is to reduce the amount of
> __get_user()/__put_user(), rather than "instrumenting" them that way.
FWIW, unsafe variants ought to be encapsulated in as few places as possible.
And that includes both unsafe_... and __... stuff. waitid() had been a dumb
fuckup (by me) and it should've been done as
static int waitid_put_siginfo(struct siginfo __user *si, struct waitid_info *info,
int signo)
{
if (!si)
return 0;
if (!access_ok(VERIFY_WRITE, si, sizeof(struct siginfo)))
return -EFAULT;
user_access_begin();
unsafe_put_user(signo, &si->si_signo, Efault);
unsafe_put_user(0, &si->si_errno, Efault);
unsafe_put_user(info->cause, &si->si_code, Efault);
unsafe_put_user(info->pid, &si->si_pid, Efault);
unsafe_put_user(info->uid, &si->si_uid, Efault);
unsafe_put_user(info->status, &si->si_status, Efault);
user_access_end();
return 0;
Efault:
user_access_end();
return -EFAULT;
}
instead, rather than mixing it with the rest. Basically, any unsafe... or __...
should be
* used as little as possible
* accompanied by access_ok() in the same function
* not mixed with other stuff within the same function
We are obviously not there yet, but __get_user()/__put_user() *are* getting killed
off.
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