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Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC
From: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 25 Jun 2014 20:20:12 +0200
Cc: LKML <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>, Alexei Starovoitov <ast@plumgrid.com>, Andrew Morton <akpm@linux-foundation.org>, Daniel Borkmann <dborkman@redhat.com>, Will Drewry <wad@chromium.org>, Julien Tinnes <jln@chromium.org>, David Drysdale <drysdale@google.com>, Linux API <linux-api@vger.kernel.org>, "x86@kernel.org" <x86@kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, linux-mips@linux-mips.org, linux-arch <linux-arch@vger.kernel.org>, linux-security-module <linux-security-module@vger.kernel.org>
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On 06/25, Kees Cook wrote:
>
> On Wed, Jun 25, 2014 at 10:24 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > However, do_execve() takes cred_guard_mutex at the start in 
> > prepare_bprm_creds()
> > and drops it in install_exec_creds(), so it should solve the problem?
>
> I can't tell yet. I'm still trying to understand the order of
> operations here. It looks like de_thread() takes the sighand lock.
> do_execve_common does:
>
> prepare_bprm_creds (takes cred_guard_mutex)
> check_unsafe_exec (checks nnp to set LSM_UNSAFE_NO_NEW_PRIVS)
> prepare_binprm (handles suid escalation, checks nnp separately)
>     security_bprm_set_creds (checks LSM_UNSAFE_NO_NEW_PRIVS)
> exec_binprm
>     load_elf_binary
>         flush_old_exec
>             de_thread (takes and releases sighand->lock)
>         install_exec_creds (releases cred_guard_mutex)

Yes, and note that when cred_guard_mutex is dropped all other threads
are already killed,

> I don't see a way to use cred_guard_mutex during tsync (which holds
> sighand->lock) without dead-locking. What were you considering here?

Just take/drop current->signal->cred_guard_mutex along with ->siglock
in seccomp_set_mode_filter() ? Unconditionally on depending on
SECCOMP_FILTER_FLAG_TSYNC.

Oleg.


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