Re: [PATCH 3/7] mm/mmap: Add IBT bitmap size to address space limit check

[Andy Lutomirski <luto@kernel.org>]

On Thu, Jun 7, 2018 at 7:42 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> The indirect branch tracking legacy bitmap takes a large address
> space.  This causes may_expand_vm() failure on the address limit
> check.  For a IBT-enabled task, add the bitmap size to the
> address limit.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
>  arch/x86/include/uapi/asm/resource.h | 5 +++++
>  include/uapi/asm-generic/resource.h  | 3 +++
>  mm/mmap.c                            | 8 +++++++-
>  3 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/uapi/asm/resource.h b/arch/x86/include/uapi/asm/resource.h
> index 04bc4db8921b..0741b2a6101a 100644
> --- a/arch/x86/include/uapi/asm/resource.h
> +++ b/arch/x86/include/uapi/asm/resource.h
> @@ -1 +1,6 @@
> +/* SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note */
> +#ifdef CONFIG_X86_INTEL_CET
> +#define rlimit_as_extra() current->thread.cet.ibt_bitmap_size
> +#endif
> +
>  #include <asm-generic/resource.h>
> diff --git a/include/uapi/asm-generic/resource.h b/include/uapi/asm-generic/resource.h
> index f12db7a0da64..8a7608a09700 100644
> --- a/include/uapi/asm-generic/resource.h
> +++ b/include/uapi/asm-generic/resource.h
> @@ -58,5 +58,8 @@
>  # define RLIM_INFINITY         (~0UL)
>  #endif
>
> +#ifndef rlimit_as_extra
> +#define rlimit_as_extra() 0
> +#endif
>
>  #endif /* _UAPI_ASM_GENERIC_RESOURCE_H */
> diff --git a/mm/mmap.c b/mm/mmap.c
> index e7d1fcb7ec58..5c07f052bed7 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3255,7 +3255,13 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>   */
>  bool may_expand_vm(struct mm_struct *mm, vm_flags_t flags, unsigned long npages)
>  {
> -       if (mm->total_vm + npages > rlimit(RLIMIT_AS) >> PAGE_SHIFT)
> +       unsigned long as_limit = rlimit(RLIMIT_AS);
> +       unsigned long as_limit_plus = as_limit + rlimit_as_extra();
> +
> +       if (as_limit_plus > as_limit)
> +               as_limit = as_limit_plus;
> +

What happens if as_limit_plus overflows?  I guess it's probably okay here.