[PATCH] mm/secretmem: Avoid letting secretmem_users drop to zero

[Kees Cook <keescook@chromium.org>]

Quoting Dmitry: "refcount_inc() needs to be done before fd_install().
After fd_install() finishes, the fd can be used by userspace and we can
have secret data in memory before the refcount_inc().

A straightforward mis-use where a user will predict the returned fd
in another thread before the syscall returns and will use it to store
secret data is somewhat dubious because such a user just shoots themself
in the foot.

But a more interesting mis-use would be to close the predicted fd and
decrement the refcount before the corresponding refcount_inc, this way
one can briefly drop the refcount to zero while there are other users
of secretmem."

Move fd_install() after refcount_inc().

Cc: Mike Rapoport <rppt@kernel.org>
Cc: Jordy Zomer <jordy@pwning.systems>
Cc: linux-mm@kvack.org
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/lkml/CACT4Y+b1sW6-Hkn8HQYw_SsT7X3tp-CJNh2ci0wG3ZnQz9jjig@mail.gmail.com
Fixes: 9a436f8ff631 ("PM: hibernate: disable when there are active secretmem users")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 mm/secretmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index 1fea68b8d5a6..924d84ba481f 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -217,8 +217,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
 
 	file->f_flags |= O_LARGEFILE;
 
-	fd_install(fd, file);
 	refcount_inc(&secretmem_users);
+	fd_install(fd, file);
 	return fd;
 
 err_put_fd:
-- 
2.30.2