Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY

From: Lokesh Gidra <lokeshgidra@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>,
	Jonathan Corbet <corbet@lwn.net>, Peter Xu <peterx@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	Eric Biggers <ebiggers@kernel.org>,
	Daniel Colascione <dancol@dancol.org>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-doc@vger.kernel.org, Kalesh Singh <kaleshsingh@google.com>,
	Calin Juravle <calin@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Jeffrey Vander Stoep <jeffv@google.com>,
	"Cc: Android Kernel" <kernel-team@android.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>, Shaohua Li <shli@fb.com>,
	Jerome Glisse <jglisse@redhat.com>,
	Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Nitin Gupta <nigupta@nvidia.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Iurii Zaikin <yzaikin@google.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	linux-mm@kvack.kernel.org, Daniel Colascione <dancol@google.com>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>
Subject: Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY
Date: Mon, 23 Nov 2020 11:17:43 -0800	[thread overview]
Message-ID: <CA+EESO7xnnJAsPneuy1dNj6F47gViGiL-z8rajY5EoGdFWs+-A@mail.gmail.com> (raw)
In-Reply-To: <20201120153337.431dc36c1975507bb1e44596@linux-foundation.org>

On Fri, Nov 20, 2020 at 3:33 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 19 Nov 2020 19:04:10 -0800 Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> > userfaultfd handles page faults from both user and kernel code.
> > Add a new UFFD_USER_MODE_ONLY flag for userfaultfd(2) that makes
> > the resulting userfaultfd object refuse to handle faults from kernel
> > mode, treating these faults as if SIGBUS were always raised, causing
> > the kernel code to fail with EFAULT.
> >
> > A future patch adds a knob allowing administrators to give some
> > processes the ability to create userfaultfd file objects only if they
> > pass UFFD_USER_MODE_ONLY, reducing the likelihood that these processes
> > will exploit userfaultfd's ability to delay kernel page faults to open
> > timing windows for future exploits.
>
> Can we assume that an update to the userfaultfd(2) manpage is in the
> works?
>
Yes, I'm working on it. Can the kernel version which will have these
patches be known now so that I can mention it in the manpage?

> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -405,6 +405,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >
> >       if (ctx->features & UFFD_FEATURE_SIGBUS)
> >               goto out;
> > +     if ((vmf->flags & FAULT_FLAG_USER) == 0 &&
> > +         ctx->flags & UFFD_USER_MODE_ONLY) {
> > +             printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd "
> > +                     "sysctl knob to 1 if kernel faults must be handled "
> > +                     "without obtaining CAP_SYS_PTRACE capability\n");
> > +             goto out;
> > +     }
> >
> >       /*
> >        * If it's already released don't get it. This avoids to loop
> > @@ -1965,10 +1972,11 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
> >       BUG_ON(!current->mm);
> >
> >       /* Check the UFFD_* constants for consistency.  */
> > +     BUILD_BUG_ON(UFFD_USER_MODE_ONLY & UFFD_SHARED_FCNTL_FLAGS);
>
> Are we sure this is true for all architectures?

Yes, none of the architectures are using the least-significant bit for
O_CLOEXEC or O_NONBLOCK.
>
> >       BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
> >       BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
> >
> > -     if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
> > +     if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | UFFD_USER_MODE_ONLY))
> >               return -EINVAL;
> >
> >       ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
> > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > index e7e98bde221f..5f2d88212f7c 100644
> > --- a/include/uapi/linux/userfaultfd.h
> > +++ b/include/uapi/linux/userfaultfd.h
> > @@ -257,4 +257,13 @@ struct uffdio_writeprotect {
> >       __u64 mode;
> >  };
> >
> > +/*
> > + * Flags for the userfaultfd(2) system call itself.
> > + */
> > +
> > +/*
> > + * Create a userfaultfd that can handle page faults only in user mode.
> > + */
> > +#define UFFD_USER_MODE_ONLY 1
> > +
> >  #endif /* _LINUX_USERFAULTFD_H */
>
> It would be nice to define this in include/linux/userfaultfd_k.h,
> alongside the other flags.  But I guess it has to be here because it's
> part of the userspace API.