On Thu, Jun 24, 2021 at 5:25 PM Ondrej Mosnacek <omosnace@xxxxxxxxxx> 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@xxxxxxxxxxxxx> > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization") > Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx> > --- > > 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(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index 14f92285d04f..24e14c36068f 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -986,7 +986,7 @@ 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; > > @@ -2088,7 +2088,7 @@ 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); > -- > 2.31.1 Ping? Any comments on this patch? -- Ondrej Mosnacek Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.
