[patch 06/11] mm/secretmem: avoid letting secretmem_users drop to zero

[Andrew Morton <akpm@linux-foundation.org>]

From: Kees Cook <keescook@chromium.org>
Subject: mm/secretmem: avoid letting secretmem_users drop to zero

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 misuse 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 misuse 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().

Link: https://lkml.kernel.org/r/20211021154046.880251-1-keescook@chromium.org
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>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Jordy Zomer <jordy@pwning.systems>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/secretmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/secretmem.c~mm-secretmem-avoid-letting-secretmem_users-drop-to-zero
+++ a/mm/secretmem.c
@@ -218,8 +218,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned i
 
 	file->f_flags |= O_LARGEFILE;
 
-	fd_install(fd, file);
 	atomic_inc(&secretmem_users);
+	fd_install(fd, file);
 	return fd;
 
 err_put_fd:
_