[RFC PATCH RESEND] userfaultfd: open userfaultfds with O_RDONLY

[Paul Moore]

On Tue, Aug 16, 2022 at 6:12 PM Paul Moore <paul at paul-moore.com> wrote:
> On Fri, Jul 8, 2022 at 5:35 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
> >
> > Since userfaultfd doesn't implement a write operation, it is more
> > appropriate to open it read-only.
> >
> > When userfaultfds are opened read-write like it is now, and such fd is
> > passed from one process to another, SELinux will check both read and
> > write permissions for the target process, even though it can't actually
> > do any write operation on the fd later.
> >
> > Inspired by the following bug report, which has hit the SELinux scenario
> > described above:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1974559
> >
> > Reported-by: Robert O'Callahan <roc at ocallahan.org>
> > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
> > Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> > ---
> >
> > Resending as the last submission was ignored for over a year...
> >
> > https://lore.kernel.org/lkml/20210624152515.1844133-1-omosnace@redhat.com/T/
> >
> > I marked this as RFC, because I'm not sure if this has any unwanted side
> > effects. I only ran this patch through selinux-testsuite, which has a
> > simple userfaultfd subtest, and a reproducer from the Bugzilla report.
> >
> > Please tell me whether this makes sense and/or if it passes any
> > userfaultfd tests you guys might have.
> >
> >  fs/userfaultfd.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
>
> VFS folks, any objection to this patch?  It seems reasonable to me and
> I'd really prefer this to go in via the vfs tree, but I'm not above
> merging this via the lsm/next tree to get someone in vfs land to pay
> attention to this ...

Okay, final warning, if I don't see any objections to this when I make
my patch sweep next week I'm going to go ahead and merge this via the
LSM tree.

> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index e943370107d0..8ccf00be63e1 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -989,7 +989,7 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *new,
> >         int fd;
> >
> >         fd = anon_inode_getfd_secure("[userfaultfd]", &userfaultfd_fops, new,
> > -                       O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS), inode);
> > +                       O_RDONLY | (new->flags & UFFD_SHARED_FCNTL_FLAGS), inode);
> >         if (fd < 0)
> >                 return fd;
> >
> > @@ -2090,7 +2090,7 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
> >         mmgrab(ctx->mm);
> >
> >         fd = anon_inode_getfd_secure("[userfaultfd]", &userfaultfd_fops, ctx,
> > -                       O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS), NULL);
> > +                       O_RDONLY | (flags & UFFD_SHARED_FCNTL_FLAGS), NULL);
> >         if (fd < 0) {
> >                 mmdrop(ctx->mm);
> >                 kmem_cache_free(userfaultfd_ctx_cachep, ctx);
> > --
> > 2.36.1
>
> --
> paul-moore.com



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