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From: Jann Horn <jannh@google.com>
To: Casey Schaufler <casey.schaufler@intel.com>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-security-module <linux-security-module@vger.kernel.org>,
	selinux@tycho.nsa.gov, Dave Hansen <dave.hansen@intel.com>,
	deneen.t.dock@intel.com, kristen@linux.intel.com,
	Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [PATCH RFC v2 2/5] X86: Support LSM determination of side-channel vulnerability
Date: Sat, 18 Aug 2018 01:55:02 +0200	[thread overview]
Message-ID: <CAG48ez23G+wcnkCMNZV_mxYMS=h+pZkpx1451Wz5GB9_tcDv3Q@mail.gmail.com> (raw)
In-Reply-To: <20180817221624.10232-3-casey.schaufler@intel.com>

On Sat, Aug 18, 2018 at 12:17 AM Casey Schaufler
<casey.schaufler@intel.com> wrote:
>
> From: Casey Schaufler <cschaufler@localhost.localdomain>
>
> When switching between tasks it may be necessary
> to set an indirect branch prediction barrier if the
> tasks are potentially vulnerable to side-channel
> attacks. This adds a call to security_task_safe_sidechannel
> so that security modules can weigh in on the decision.
>
> Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
> ---
>  arch/x86/mm/tlb.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 6eb1f34c3c85..8714d4af06aa 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -7,6 +7,7 @@
>  #include <linux/export.h>
>  #include <linux/cpu.h>
>  #include <linux/debugfs.h>
> +#include <linux/security.h>
>
>  #include <asm/tlbflush.h>
>  #include <asm/mmu_context.h>
> @@ -270,11 +271,14 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>                  * threads. It will also not flush if we switch to idle
>                  * thread and back to the same process. It will flush if we
>                  * switch to a different non-dumpable process.
> +                * If a security module thinks that the transition
> +                * is unsafe do the flush.
>                  */
> -               if (tsk && tsk->mm &&
> -                   tsk->mm->context.ctx_id != last_ctx_id &&
> -                   get_dumpable(tsk->mm) != SUID_DUMP_USER)
> -                       indirect_branch_prediction_barrier();
> +               if (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id) {
> +                       if (get_dumpable(tsk->mm) != SUID_DUMP_USER ||
> +                           security_task_safe_sidechannel(tsk) != 0)
> +                               indirect_branch_prediction_barrier();
> +               }

When you posted v1 of this series, I asked:

| Does this enforce transitivity? What happens if we first switch from
| an attacker task to a task without ->mm, and immediately afterwards
| from the task without ->mm to a victim task? In that case, whether a
| flush happens between the attacker task and the victim task depends on
| whether the LSM thinks that the mm-less task should have access to the
| victim task, right?

Have you addressed that? I don't see it...

  reply	other threads:[~2018-08-17 23:55 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-17 22:16 [PATCH RFC v2 0/5] LSM: Add and use a hook for side-channel safety checks Casey Schaufler
2018-08-17 22:16 ` [PATCH RFC v2 1/5] LSM: Introduce a hook for side-channel danger Casey Schaufler
2018-08-17 22:16 ` [PATCH RFC v2 2/5] X86: Support LSM determination of side-channel vulnerability Casey Schaufler
2018-08-17 23:55   ` Jann Horn [this message]
2018-08-20 14:45     ` Schaufler, Casey
2018-08-21 10:20       ` Jann Horn
2018-08-21 16:37         ` Schaufler, Casey
2018-08-21 17:45           ` Jann Horn
2018-08-17 22:16 ` [PATCH RFC v2 3/5] LSM: Security module checking for side-channel dangers Casey Schaufler
2018-08-17 23:52   ` Jann Horn
2018-08-20 15:31     ` Schaufler, Casey
2018-08-17 22:16 ` [PATCH RFC v2 4/5] Smack: Support determination of side-channel vulnerability Casey Schaufler
2018-08-17 22:16 ` [PATCH RFC v2 5/5] SELinux: Support SELinux " Casey Schaufler
2018-08-20 16:02   ` Stephen Smalley
2018-08-20 16:59     ` Schaufler, Casey
2018-08-20 17:43       ` Stephen Smalley
2018-08-20 19:30         ` Schaufler, Casey

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