Re: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to manufacture shadow stack PTEs
From: Deepak Gupta
Date: Mon Feb 13 2023 - 15:01:19 EST
On Mon, Feb 13, 2023 at 03:56:22PM +0100, David Hildenbrand wrote:
On 13.02.23 15:37, Deepak Gupta wrote:
On Mon, Feb 13, 2023 at 01:05:16PM +0100, David Hildenbrand wrote:
On 13.02.23 05:53, Deepak Gupta wrote:
maybe_mkwrite creates PTEs with WRITE encodings for underlying arch if
VM_WRITE is turned on in vma->vm_flags. Shadow stack memory is a write-
able memory except it can only be written by certain specific
instructions. This patch allows maybe_mkwrite to create shadow stack PTEs
if vma is shadow stack VMA. Each arch can define which combination of VMA
flags means a shadow stack.
Additionally pte_mkshdwstk must be provided by arch specific PTE
construction headers to create shadow stack PTEs. (in arch specific
pgtable.h).
This patch provides dummy/stub pte_mkshdwstk if CONFIG_USER_SHADOW_STACK
is not selected.
Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
---
include/linux/mm.h | 23 +++++++++++++++++++++--
include/linux/pgtable.h | 4 ++++
2 files changed, 25 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8f857163ac89..a7705bc49bfe 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1093,6 +1093,21 @@ static inline unsigned long thp_size(struct page *page)
void free_compound_page(struct page *page);
#ifdef CONFIG_MMU
+
+#ifdef CONFIG_USER_SHADOW_STACK
+bool arch_is_shadow_stack_vma(struct vm_area_struct *vma);
+#endif
+
+static inline bool
+is_shadow_stack_vma(struct vm_area_struct *vma)
+{
+#ifdef CONFIG_USER_SHADOW_STACK
+ return arch_is_shadow_stack_vma(vma);
+#else
+ return false;
+#endif
+}
+
/*
* Do pte_mkwrite, but only if the vma says VM_WRITE. We do this when
* servicing faults for write access. In the normal case, do always want
@@ -1101,8 +1116,12 @@ void free_compound_page(struct page *page);
*/
static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
- if (likely(vma->vm_flags & VM_WRITE))
- pte = pte_mkwrite(pte);
+ if (likely(vma->vm_flags & VM_WRITE)) {
+ if (unlikely(is_shadow_stack_vma(vma)))
+ pte = pte_mkshdwstk(pte);
+ else
+ pte = pte_mkwrite(pte);
+ }
return pte;
Exactly what we are trying to avoid in the x86 approach right now.
Please see the x86 series on details, we shouldn't try reinventing the
wheel but finding a core-mm approach that fits multiple architectures.
https://lkml.kernel.org/r/20230119212317.8324-1-rick.p.edgecombe@xxxxxxxxx
Thanks David for comment here. I looked at x86 approach. This patch
actually written in a way which is not re-inventing wheel and is following
a core-mm approach that fits multiple architectures.
Change above checks `is_shadow_stack_vma` and if it returns true then only
it manufactures shadow stack pte else it'll make a regular writeable mapping.
Now if we look at `is_shadow_stack_vma` implementation, it returns false if
`CONFIG_USER_SHADOW_STACK` is not defined. If `CONFIG_USER_SHADOW_STACK is
defined then it calls `arch_is_shadow_stack_vma` which should be implemented
by arch specific code. This allows each architecture to define their own vma
flag encodings for shadow stack (riscv chooses presence of only `VM_WRITE`
which is analogous to choosen PTE encodings on riscv W=1,R=0,X=0)
Additionally pte_mkshdwstk will be nop if not implemented by architecture.
Let me know if this make sense. If I am missing something here, let me know.
See the discussion in that thread. The idea is to pass a VMA to
pte_mkwrite() and let it handle how to actually set it writable.
Thanks. I see. Instances where `pte_mkwrite` is directly invoked by checking
VM_WRITE and thus instead of fixing all those instance, make pte_mkwrite itself
take vma flag or vma.
I'll revise.
--
Thanks,
David / dhildenb