Re: [PATCH 2/2] x86/mm/iommu/sva: Do not allow to set FORCE_TAGGED_SVA bit from outside

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, 3 Apr 2023 at 13:10, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> arch_prctl(ARCH_FORCE_TAGGED_SVA) overrides the default and allows LAM
> and SVA to co-exist in the process. It is expected by called by the
> process when it knows what it is doing.
>
> arch_prctl() operates on the current process, but the same code is
> reachable from ptrace where it can be called on arbitrary task.
>
> Make it strict and only allow to set MM_CONTEXT_FORCE_TAGGED_SVA for the
> current process.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: 23e5d9ec2bab ("x86/mm/iommu/sva: Make LAM and SVA mutually exclusive")
> Suggested-by: Dmitry Vyukov <dvyukov@google.com>
> ---
>  arch/x86/kernel/process_64.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index c7dfd727c9ec..cefac2d3a9f6 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -885,6 +885,8 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
>         case ARCH_ENABLE_TAGGED_ADDR:
>                 return prctl_enable_tagged_addr(task->mm, arg2);
>         case ARCH_FORCE_TAGGED_SVA:
> +               if (current != task)
> +                       return -EINVAL;

prctl_enable_tagged_addr() checks "task->mm != current->mm".
Should we check the same here for consistency? Or also change the
check in prctl_enable_tagged_addr().

arch_prctl() can only do task==current, so I guess "current != task"
is a more reasonable check for prctl_enable_tagged_addr() as well.



>                 set_bit(MM_CONTEXT_FORCE_TAGGED_SVA, &task->mm->context.flags);
>                 return 0;
>         case ARCH_GET_MAX_TAG_BITS:
> --
> 2.39.2
>