Re: [PATCH v3 09/29] mm: abstract shadow stack vma behind `vma_is_shadow_stack`

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On 04.04.24 01:34, Deepak Gupta wrote:
VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) to encode shadow stack VMA.

This patch changes checks of VM_SHADOW_STACK flag in generic code to call
to a function `vma_is_shadow_stack` which will return true if its a
shadow stack vma and default stub (when support doesnt exist) returns false.

Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
Suggested-by: Mike Rapoport <rppt@xxxxxxxxxx>
---
  include/linux/mm.h | 13 ++++++++++++-
  mm/gup.c           |  5 +++--
  mm/internal.h      |  2 +-
  3 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 64109f6c70f5..9952937be659 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -363,8 +363,19 @@ extern unsigned int kobjsize(const void *objp);
#ifndef VM_SHADOW_STACK
  # define VM_SHADOW_STACK	VM_NONE
+
+static inline bool vma_is_shadow_stack(vm_flags_t vm_flags)
+{
+	return false;
+}
+#else
+static inline bool vma_is_shadow_stack(vm_flags_t vm_flags)
+{
+	return (vm_flags & VM_SHADOW_STACK);
+}
  #endif

You can simply do outside the ifdef

static inline bool vma_is_shadow_stack(vm_flags_t vm_flags)
{
	return !!(vm_flags & VM_SHADOW_STACK);
}

This will work even when VM_SHADOW_STACK is defined to be VM_NONE.

+

unrelated code change

  #if defined(CONFIG_X86)
  # define VM_PAT		VM_ARCH_1	/* PAT reserves whole VMA at once (x86) */
  #elif defined(CONFIG_PPC)
@@ -3473,7 +3484,7 @@ static inline unsigned long stack_guard_start_gap(struct vm_area_struct *vma)
  		return stack_guard_gap;
/* See reasoning around the VM_SHADOW_STACK definition */
-	if (vma->vm_flags & VM_SHADOW_STACK)
+	if (vma->vm_flags && vma_is_shadow_stack(vma->vm_flags))

Pretty sure:

if (vma_is_shadow_stack(vma->vm_flags))

  		return PAGE_SIZE;
return 0;
diff --git a/mm/gup.c b/mm/gup.c
index df83182ec72d..a7a02eb0a6b3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1053,7 +1053,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
  		    !writable_file_mapping_allowed(vma, gup_flags))
  			return -EFAULT;
- if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
+		if (!(vm_flags & VM_WRITE) || vma_is_shadow_stack(vm_flags)) {
  			if (!(gup_flags & FOLL_FORCE))
  				return -EFAULT;
  			/* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
@@ -1071,7 +1071,8 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
  			if (!is_cow_mapping(vm_flags))
  				return -EFAULT;
  		}
-	} else if (!(vm_flags & VM_READ)) {
+	} else if (!(vm_flags & VM_READ) && !vma_is_shadow_stack(vm_flags)) {
+	/* reads allowed if its shadow stack vma */
  		if (!(gup_flags & FOLL_FORCE))
  			return -EFAULT;
  		/*

Unless I am missing something, this is not a simple cleanup. It should go into a separate patch with a clearly documented reason for that change.

--
Cheers,

David / dhildenb





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