Re: [PATCH 1/2] Add UFFD_USER_MODE_ONLY

[Lokesh Gidra]

On Fri, Jul 24, 2020 at 7:28 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Wed, Apr 22, 2020 at 05:26:31PM -0700, Daniel Colascione 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.
> >
> > Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx>
>
> Something to add here is that there is separate work on selinux to
> support limiting specific userspace programs to only this type of
> userfaultfd.
>
> I also think Kees' comment about documenting what is the threat being solved
> including some links to external sources still applies.
>
> Finally, a question:
>
> Is there any way at all to increase security without breaking
> the assumption that copy_from_user is the same as userspace read?
>
>
> As an example of a drastical approach that might solve some issues, how
> about allocating some special memory and setting some VMA flag, then
> limiting copy from/to user to just this subset of virtual addresses?
> We can then do things like pin these pages in RAM, forbid
> madvise/userfaultfd for these addresses, etc.
>
> Affected userspace then needs to use a kind of a bounce buffer for any
> calls into kernel.  This needs much more support from userspace and adds
> much more overhead, but on the flip side, affects more ways userspace
> can slow down the kernel.
>
> Was this discussed in the past? Links would be appreciated.
>
Adding Nick and Jeff to the discussion.
>
> > ---
> >  fs/userfaultfd.c                 | 7 ++++++-
> >  include/uapi/linux/userfaultfd.h | 9 +++++++++
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index e39fdec8a0b0..21378abe8f7b 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -418,6 +418,9 @@ 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)
> > +             goto out;
> >
> >       /*
> >        * If it's already released don't get it. This avoids to loop
> > @@ -2003,6 +2006,7 @@ static void init_once_userfaultfd_ctx(void *mem)
> >
> >  SYSCALL_DEFINE1(userfaultfd, int, flags)
> >  {
> > +     static const int uffd_flags = UFFD_USER_MODE_ONLY;
> >       struct userfaultfd_ctx *ctx;
> >       int fd;
> >
> > @@ -2012,10 +2016,11 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
> >       BUG_ON(!current->mm);
> >
> >       /* Check the UFFD_* constants for consistency.  */
> > +     BUILD_BUG_ON(uffd_flags & UFFD_SHARED_FCNTL_FLAGS);
> >       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_flags))
> >               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 */
> > --
> > 2.26.2.303.gf8c07b1a785-goog
> >
>