Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY

[Lokesh Gidra <lokeshgidra@google.com>]

On Thu, Nov 19, 2020 at 7:04 PM 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.
>
> Signed-off-by: Daniel Colascione <dancol@google.com>
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  fs/userfaultfd.c                 | 10 +++++++++-
>  include/uapi/linux/userfaultfd.h |  9 +++++++++
>  2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 000b457ad087..605599fde015 100644
> --- 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);
>         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 */
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>
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