linux-kernel - Re: Kernel panic at Ubuntu: IMA + Apparmor

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Date:	Tue, 29 Apr 2014 09:00:12 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Dmitry Kasatkin <d.kasatkin@...sung.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	John Johansen <john.johansen@...onical.com>,
	James Morris <jmorris@...ei.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernel-team <kernel-team@...ts.ubuntu.com>,
	Eric Paris <eparis@...hat.com>
Subject: Re: Kernel panic at Ubuntu: IMA + Apparmor

My apologies for those receiving this post a 2nd time.  The original
post never made it the mailing lists ...

On Fri, 2014-04-25 at 15:25 -0700, Eric W. Biederman wrote: 
> Al Viro <viro@...IV.linux.org.uk> writes:
> 
> > On Fri, Apr 25, 2014 at 02:43:42PM -0700, Eric W. Biederman wrote:
> >
> >> ssize_t __vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
> >> {
> >> 	ssize_t ret;
> >> 
> >> 	if (!(file->f_mode & FMODE_READ))
> >> 		return -EBADF;
> >> 	if (!file->f_op->read && !file->f_op->aio_read)
> >> 		return -EINVAL;
> >> 	if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
> >> 		return -EFAULT;
> >> 
> >> 	if (ret >= 0) {
> >> 		count = ret;
> >> 		if (file->f_op->read)
> >> 			ret = file->f_op->read(file, buf, count, pos);
> >> 		else
> >> 			ret = do_sync_read(file, buf, count, pos);
> >> 	}
> >> 
> >> 	return ret;
> >> }
> >
> > ... which lacks the f_pos wraparound, etc. checks done by rw_verify_area().
> > IOW, it's one more place to grep through while verifying that ->read()
> > et.al. do not get called with such arguments.
> 
> Agreed it must be done more delicately than my sketch.  I am not
> familiar with how much value such sanity checks add.  Especially when
> the read is not coming from a potentially hostile userspace.

Sorry for the delay in commenting, imap problems.  This sounds like a
plausible solution, similar to __vfs_setxattr_noperm() vs.
__vfs_setxattr().

> > fanotify probably could be skipped - ask the security circus crowd about
> > that one, it's their bast^Wbaby.  
> 
> When the point is having a factor of read that skips the security circus
> I think it makes sense to skip this too.  At least as a starting
> position.

Right, fsnotify*() is meant for userspace access, not kernel access.
CC'ing Eric Paris for comment.

> > add_rchar() and inc_syscr()... depends on
> > whether you want those reads hidden from accounting.
> 
> I doubt it matters in practice, the code is cheap.
> 
> Still it feels wrong to account reads to a task that did not ask for
> them.  It feels more correct to account that kind of read into a
> different bucket.  Say the reads performed by the kernel for mysterious
> kernel activities.

Ok. So who are the interested parties that need to be included in this
discussion?

thanks,

Mimi

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