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Message-ID: <CAG48ez0=RSok9mAo9QHsho+Qnm_ZDqkH68Amfzj9A_jWmOZaOA@mail.gmail.com>
Date: Thu, 16 Aug 2018 16:09:31 +0200
From: Jann Horn <jannh@...gle.com>
To: casey.schaufler@...el.com
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
kernel list <linux-kernel@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>, selinux@...ho.nsa.gov,
SMACK-discuss@...ts.01.org, Dave Hansen <dave.hansen@...el.com>, deneen.t.dock@...el.com,
kristen@...ux.intel.com, Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH RFC 2/5] X86: Support LSM determination of side-channel vulnerability
On Thu, Aug 16, 2018 at 11:51 AM Casey Schaufler
<casey.schaufler@...el.com> wrote:
>
> From: Casey Schaufler <cschaufler@...alhost.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@...el.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();
> + }
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?
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