On Thu, 21 Oct 2021 at 17:40, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > 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@xxxxxxxxxx> > Cc: Jordy Zomer <jordy@pwning.systems> > Cc: linux-mm@xxxxxxxxx > Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> > Link: https://lore.kernel.org/lkml/CACT4Y+b1sW6-Hkn8HQYw_SsT7X3tp-CJNh2ci0wG3ZnQz9jjig@xxxxxxxxxxxxxx > Fixes: 9a436f8ff631 ("PM: hibernate: disable when there are active secretmem users") > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> > --- > 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 >
