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Thu, 21 Oct 2021 08:49:07 -0700 (PDT) MIME-Version: 1.0 References: <20211021154046.880251-1-keescook@chromium.org> In-Reply-To: <20211021154046.880251-1-keescook@chromium.org> From: Dmitry Vyukov Date: Thu, 21 Oct 2021 17:48:55 +0200 Message-ID: Subject: Re: [PATCH] mm/secretmem: Avoid letting secretmem_users drop to zero To: Kees Cook Cc: Andrew Morton , Mike Rapoport , Jordy Zomer , linux-mm@kvack.org, James Bottomley , David Hildenbrand , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 05005F0003A5 X-Stat-Signature: pm5uawbj73ztxqnybjh3y697gi6uchpm Authentication-Results: imf17.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b="jiZK/wFz"; spf=pass (imf17.hostedemail.com: domain of dvyukov@google.com designates 209.85.210.42 as permitted sender) smtp.mailfrom=dvyukov@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1634831347-618040 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, 21 Oct 2021 at 17:40, Kees Cook 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 > Cc: Jordy Zomer > Cc: linux-mm@kvack.org > Reported-by: Dmitry Vyukov > 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 Reviewed-by: Dmitry Vyukov > --- > 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 >