From 855d44434fa24d5344c1cb0edb38723f891cd415 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 28 Oct 2021 14:36:21 -0700 Subject: [PATCH] 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 Reported-by: Dmitry Vyukov Reviewed-by: Dmitry Vyukov Reviewed-by: David Hildenbrand Reviewed-by: Jordy Zomer Cc: Mike Rapoport Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/secretmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) 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: