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

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On 25.01.24 07:21, debug@xxxxxxxxxxxx wrote:
From: Deepak Gupta <debug@xxxxxxxxxxxx>

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@xxxxxxxxxxxx>
---
  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.

--
Cheers,

David / dhildenb





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