linux-kernel - Re: [PATCH 3/3] userfaultfd: apply unprivileged

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Date:   Mon, 11 Mar 2019 17:58:40 +0800
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        kvm@...r.kernel.org, Jerome Glisse <jglisse@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>, linux-mm@...ck.org,
        Marty McFadden <mcfadden8@...l.gov>,
        Maya Gokhale <gokhale2@...l.gov>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        linux-fsdevel@...r.kernel.org,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] userfaultfd: apply unprivileged_userfaultfd check

On Mon, Mar 11, 2019 at 05:37:01PM +0800, Peter Xu wrote:
> Apply the unprivileged_userfaultfd check when doing userfaultfd
> syscall.  We didn't check it in other paths of userfaultfd (e.g., the
> ioctl() path) because we don't want to drag down the fast path of
> userfaultfd, as suggested by Andrea.
> 
> Suggested-by: Andrea Arcangeli <aarcange@...hat.com>
> Suggested-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  fs/userfaultfd.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index c2188464555a..effdcfc88629 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -951,6 +951,28 @@ void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
>  	}
>  }
>  
> +/* Whether current process allows to use userfaultfd syscalls */
> +static bool userfaultfd_allowed(void)
> +{
> +	bool allowed = false;
> +
> +	switch (unprivileged_userfaultfd) {
> +	case UFFD_UNPRIV_ENABLED:
> +		allowed = true;
> +		break;
> +	case UFFD_UNPRIV_KVM:
> +		allowed = !!test_bit(MMF_USERFAULTFD_ALLOW,
> +				     &current->mm->flags);
> +		/* Fall through */

Sorry I should squash this in otherwise compilation of !CONFIG_KVM
will break:

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index effdcfc88629..1b3fa5935643 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -960,10 +960,12 @@ static bool userfaultfd_allowed(void)
        case UFFD_UNPRIV_ENABLED:
                allowed = true;
                break;
+#if IS_ENABLED(CONFIG_KVM)
        case UFFD_UNPRIV_KVM:
                allowed = !!test_bit(MMF_USERFAULTFD_ALLOW,
                                     &current->mm->flags);
                /* Fall through */
+#endif
        case UFFD_UNPRIV_DISABLED:
                allowed = allowed || ns_capable(current_user_ns(),
                                                CAP_SYS_PTRACE);

Will wait for more comments before I repost.  Sorry for the noise.

Regards,

-- 
Peter Xu