[PATCH v3 13/29] riscv mmu: write protect and shadow stack

[Alexandre Ghiti]

Hi Deepak,

On Mon, May 13, 2024 at 7:32 PM Deepak Gupta <debug at rivosinc.com> wrote:
>
> On Sun, May 12, 2024 at 06:31:24PM +0200, Alexandre Ghiti wrote:
> >On 04/04/2024 01:35, Deepak Gupta wrote:
> >>`fork` implements copy on write (COW) by making pages readonly in child
> >>and parent both.
> >>
> >>ptep_set_wrprotect and pte_wrprotect clears _PAGE_WRITE in PTE.
> >>Assumption is that page is readable and on fault copy on write happens.
> >>
> >>To implement COW on such pages,
> >
> >
> >I guess you mean "shadow stack pages" here.
>
> Yes I meant shadow stack pages. Will fix the message.
>
> >
> >
> >>  clearing up W bit makes them XWR = 000.
> >>This will result in wrong PTE setting which says no perms but V=1 and PFN
> >>field pointing to final page. Instead desired behavior is to turn it into
> >>a readable page, take an access (load/store) fault on sspush/sspop
> >>(shadow stack) and then perform COW on such pages.
> >>This way regular reads
> >>would still be allowed and not lead to COW maintaining current behavior
> >>of COW on non-shadow stack but writeable memory.
> >>
> >>On the other hand it doesn't interfere with existing COW for read-write
> >>memory. Assumption is always that _PAGE_READ must have been set and thus
> >>setting _PAGE_READ is harmless.
> >>
> >>Signed-off-by: Deepak Gupta <debug at rivosinc.com>
> >>---
> >>  arch/riscv/include/asm/pgtable.h | 12 ++++++++++--
> >>  1 file changed, 10 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> >>index 9b837239d3e8..7a1c2a98d272 100644
> >>--- a/arch/riscv/include/asm/pgtable.h
> >>+++ b/arch/riscv/include/asm/pgtable.h
> >>@@ -398,7 +398,7 @@ static inline int pte_special(pte_t pte)
> >>  static inline pte_t pte_wrprotect(pte_t pte)
> >>  {
> >>-     return __pte(pte_val(pte) & ~(_PAGE_WRITE));
> >>+     return __pte((pte_val(pte) & ~(_PAGE_WRITE)) | (_PAGE_READ));
> >>  }
> >>  /* static inline pte_t pte_mkread(pte_t pte) */
> >>@@ -581,7 +581,15 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> >>  static inline void ptep_set_wrprotect(struct mm_struct *mm,
> >>                                    unsigned long address, pte_t *ptep)
> >>  {
> >>-     atomic_long_and(~(unsigned long)_PAGE_WRITE, (atomic_long_t *)ptep);
> >>+     volatile pte_t read_pte = *ptep;

Sorry I missed this ^. You need to use ptep_get() to get the value of
a pte. And why do you need the volatile here?

> >>+     /*
> >>+      * ptep_set_wrprotect can be called for shadow stack ranges too.
> >>+      * shadow stack memory is XWR = 010 and thus clearing _PAGE_WRITE will lead to
> >>+      * encoding 000b which is wrong encoding with V = 1. This should lead to page fault
> >>+      * but we dont want this wrong configuration to be set in page tables.
> >>+      */
> >>+     atomic_long_set((atomic_long_t *)ptep,
> >>+                     ((pte_val(read_pte) & ~(unsigned long)_PAGE_WRITE) | _PAGE_READ));
> >>  }
> >>  #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
> >
> >
> >Doesn't making the shadow stack page readable allow "normal" loads to
> >access the page? If it does, isn't that an issue (security-wise)?
>
> When shadow stack permissions are there (i.e. R=0, W=1, X=0), then also shadow stack is
> readable through "normal" loads. So nothing changes when it converts into a readonly page
> from page permissions perspective.
>
> Security-wise it's not a concern because from threat modeling perspective, if attacker had
> read-write primitives (via some bug in program) available to read and write address space
> of process/task; then they would have availiblity of return addresses on normal stack. It's
> the write primitive that is concerning and to be protected against. And that's why shadow stack
> is not writeable using "normal" stores.
>
> >

Thanks for the explanation!

With the use of ptep_get(), you can add:

Reviewed-by: Alexandre Ghiti <alexghiti at rivosinc.com>

Thanks,

Alex