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/[email protected]
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 <[email protected]>
Reported-by: Dmitry Vyukov <[email protected]>
Reviewed-by: Dmitry Vyukov <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Jordy Zomer <[email protected]>
Cc: Mike Rapoport <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/secretmem.c b/mm/secretmem.c
index c2dda408bb36..22b310adb53d 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -218,8 +218,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
 
 	file->f_flags |= O_LARGEFILE;
 
-	fd_install(fd, file);
 	atomic_inc(&secretmem_users);
+	fd_install(fd, file);
 	return fd;
 
 err_put_fd: