linux-kernel - Re: [PATCH v6 22/41] mm/mmap: Add shadow stack pages to memory accounting

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Date:   Tue, 21 Feb 2023 09:31:10 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
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Cc:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v6 22/41] mm/mmap: Add shadow stack pages to memory
 accounting

On 20.02.23 23:44, Edgecombe, Rick P wrote:
> On Mon, 2023-02-20 at 13:58 +0100, David Hildenbrand wrote:
>> On 18.02.23 22:14, Rick Edgecombe wrote:
>>> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
>>>
>>> The x86 Control-flow Enforcement Technology (CET) feature includes
>>> a new
>>> type of memory called shadow stack. This shadow stack memory has
>>> some
>>> unusual properties, which requires some core mm changes to function
>>> properly.
>>>
>>> Account shadow stack pages to stack memory.
>>>
>>> Reviewed-by: Kees Cook <keescook@...omium.org>
>>> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
>>> Tested-by: John Allen <john.allen@....com>
>>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
>>> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
>>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
>>> Cc: Kees Cook <keescook@...omium.org>
>>>
>>> ---
>>> v3:
>>>     - Remove unneeded VM_SHADOW_STACK check in accountable_mapping()
>>>       (Kirill)
>>>
>>> v2:
>>>     - Remove is_shadow_stack_mapping() and just change it to
>>> directly bitwise
>>>       and VM_SHADOW_STACK.
>>>
>>> Yu-cheng v26:
>>>     - Remove redundant #ifdef CONFIG_MMU.
>>>
>>> Yu-cheng v25:
>>>     - Remove #ifdef CONFIG_ARCH_HAS_SHADOW_STACK for
>>> is_shadow_stack_mapping().
>>> ---
>>>     mm/mmap.c | 2 ++
>>>     1 file changed, 2 insertions(+)
>>>
>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>> index 425a9349e610..9f85596cce31 100644
>>> --- a/mm/mmap.c
>>> +++ b/mm/mmap.c
>>> @@ -3290,6 +3290,8 @@ void vm_stat_account(struct mm_struct *mm,
>>> vm_flags_t flags, long npages)
>>>                 mm->exec_vm += npages;
>>>         else if (is_stack_mapping(flags))
>>>                 mm->stack_vm += npages;
>>> +     else if (flags & VM_SHADOW_STACK)
>>> +             mm->stack_vm += npages;
>>
>> Why not modify is_stack_mapping() ?
> 
> It kind of sticks out a little in this conditional, but
> is_stack_mapping() has this comment:
> /*
>   * Stack area - automatically grows in one direction
>   *
>   * VM_GROWSUP / VM_GROWSDOWN VMAs are always private anonymous:
>   * do_mmap() forbids all other combinations.
>   */
> 
> Shadow stack don't grow, so it doesn't quite fit. There used to be an
> is_shadow_stack_mapping(), but it was removed because all that was
> needed (for the time being) was the simple bitwise AND:
> 
> https://lore.kernel.org/lkml/804adbac-61e6-0fd2-f726-5735fb290199@intel.com/

As there is only a single user of is_stack_mapping(), I'd simply have 
adjusted the doc of is_stack_mapping() to include shadow stacks.

-- 
Thanks,

David / dhildenb