linux-kernel - Re: [RFC PATCH v1 09/28] mm: abstract shadow stack vma behind `arch_is_shadow

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Date: Tue, 13 Feb 2024 11:34:59 +0100
From: David Hildenbrand <david@...hat.com>
To: Deepak Gupta <debug@...osinc.com>
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Subject: Re: [RFC PATCH v1 09/28] mm: abstract shadow stack vma behind
 `arch_is_shadow_stack`

On 25.01.24 18:07, Deepak Gupta wrote:
> On Thu, Jan 25, 2024 at 09:18:07AM +0100, David Hildenbrand wrote:
>> On 25.01.24 07:21, debug@...osinc.com wrote:
>>> From: Deepak Gupta <debug@...osinc.com>
>>>
>>> x86 has used VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) to encode shadow
>>> stack VMA. VM_SHADOW_STACK is thus not possible on 32bit. Some arches may
>>> need a way to encode shadow stack on 32bit and 64bit both and they may
>>> encode this information differently in VMAs.
>>>
>>> This patch changes checks of VM_SHADOW_STACK flag in generic code to call
>>> to a function `arch_is_shadow_stack` which will return true if arch
>>> supports shadow stack and vma is shadow stack else stub returns false.
>>>
>>> There was a suggestion to name it as `vma_is_shadow_stack`. I preferred to
>>> keep `arch` prefix in there because it's each arch specific.
>>>
>>> Signed-off-by: Deepak Gupta <debug@...osinc.com>
>>> ---
>>>   include/linux/mm.h | 18 +++++++++++++++++-
>>>   mm/gup.c           |  5 +++--
>>>   mm/internal.h      |  2 +-
>>>   3 files changed, 21 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index dfe0e8118669..15c70fc677a3 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -352,6 +352,10 @@ extern unsigned int kobjsize(const void *objp);
>>>    * for more details on the guard size.
>>>    */
>>>   # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
>>> +static inline bool arch_is_shadow_stack(vm_flags_t vm_flags)
>>> +{
>>> +	return (vm_flags & VM_SHADOW_STACK);
>>> +}
>>>   #endif
>>>   #ifdef CONFIG_RISCV_USER_CFI
>>> @@ -362,10 +366,22 @@ extern unsigned int kobjsize(const void *objp);
>>>    * with VM_SHARED.
>>>    */
>>>   #define VM_SHADOW_STACK	VM_WRITE
>>> +
>>> +static inline bool arch_is_shadow_stack(vm_flags_t vm_flags)
>>> +{
>>> +	return ((vm_flags & (VM_WRITE | VM_READ | VM_EXEC)) == VM_WRITE);
>>> +}
>>> +
>>
>> Please no such hacks just to work around the 32bit vmflags limitation.
> 
> As I said in another response. Noted.
> And if there're no takers for 32bit on riscv (which highly likely is the case)
> This will go away in next version of patchsets.

Sorry for the (unusually for me) late reply. Simplifying to riscv64 
sounds great.

Alternatively, maybe VM_SHADOW_STACK is not even required at all on 
riscv if we can teach all code to only stare at arch_is_shadow_stack() 
instead.

.. but, just using the same VM_SHADOW_STACK will it all much cleaner. 
Eventually, we can just stop playing arch-specific games with 
arch_is_shadow_stack and VM_SHADOW_STACK.

-- 
Cheers,

David / dhildenb