Re: [PATCH RFC 2/5] X86: Support LSM determination of side-channel vulnerability

[Jann Horn]

On Thu, Aug 16, 2018 at 11:51 AM Casey Schaufler
<casey.schaufler@xxxxxxxxx> wrote:
>
> From: Casey Schaufler <cschaufler@xxxxxxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxx>
> ---
>  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?