From: Ondrej Mosnacek omosnace@redhat.com
[ 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 --- fs/userfaultfd.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 175de70e3adf..0c1d33c4f74c 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -991,7 +991,7 @@ 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;
@@ -2094,7 +2094,7 @@ 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);