From 4436769c4488692c89780296bc5159643e3a20e4 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook@chromium.org>
Date: Thu, 28 Oct 2021 14:36:21 -0700
Subject: [PATCH] mm/secretmem: avoid letting secretmem_users drop to zero

References: git fixes (mm/secretmem)
Patch-mainline: v5.15
Git-commit: 855d44434fa24d5344c1cb0edb38723f891cd415

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/secretmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index 030f02ddc7c1..b9f5826a8190 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -216,8 +216,8 @@ 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: