On Fri, Nov 20, 2020 at 3:33 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 19 Nov 2020 19:04:10 -0800 Lokesh Gidra <lokeshgidra@xxxxxxxxxx> 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.
