userfaultfd: open userfaultfds with O_RDONLY

[ Upstream commit abec3d015fdfb7c63105c7e1c956188bf381aa55 ]

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@ocallahan.org>
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 22bf14ab2d163b189ba3f9a3fedc5fe53ec7bccb..b56e8e31d967fb9f17cb1e34489205755cbc99ff 100644 (file)
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -982,7 +982,7 @@ 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;
 
@@ -2097,7 +2097,7 @@ 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);