Re: [RFC PATCH v1 09/28] mm: abstract shadow stack vma behind `arch_is_shadow_stack`

Deepak Gupta
2024-01-25T17:07:08Z

On Thu, Jan 25, 2024 at 09:18:07AM +0100, David Hildenbrand wrote:
>On 25.01.24 07:21, debug@rivosinc.com wrote:
>>From: Deepak Gupta <debug@rivosinc.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@rivosinc.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.

>
>--
>Cheers,
>
>David / dhildenb
>