Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

Yu-cheng Yu
2020-09-11T23:01:20Z

On Wed, 2020-09-09 at 16:29 -0700, Dave Hansen wrote:
> On 9/9/20 4:25 PM, Yu, Yu-cheng wrote:
> > On 9/9/2020 4:11 PM, Dave Hansen wrote:
> > > On 9/9/20 4:07 PM, Yu, Yu-cheng wrote:
> > > > What if a writable mapping is passed to madvise(MADV_SHSTK)? Should
> > > > that be rejected?
> > >
> > > It doesn't matter to me. Even if it's readable, it _stops_ being even
> > > directly readable after it's a shadow stack, right? I don't think
> > > writes are special in any way. If anything, we *want* it to be writable
> > > because that indicates that it can be written to, and we will want to
> > > write to it soon.
> > >
> > But in a PROT_WRITE mapping, all the pte's have _PAGE_BIT_RW set. To
> > change them to shadow stack, we need to clear that bit from the pte's.
> > That will be like mprotect_fixup()/change_protection_range().
>
> The page table hardware bits don't matter. The user-visible protection
> effects matter.
>
> For instance, we have PROT_EXEC, which *CLEARS* a hardware NX PTE bit.
> The PROT_ permissions are independent of the hardware.
>
> I don't think the interface should be influenced at *all* by what whacko
> PTE bit combinations we have to set to get the behavior.

Here are the changes if we take the mprotect(PROT_SHSTK) approach.
Any comments/suggestions?

---
arch/x86/include/uapi/asm/mman.h | 26 +++++++++++++++++++++++++-
mm/mprotect.c | 11 +++++++++++
2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
index d4a8d0424bfb..024f006fcfe8 100644
--- a/arch/x86/include/uapi/asm/mman.h
+++ b/arch/x86/include/uapi/asm/mman.h
@@ -4,6 +4,8 @@

#define MAP_32BIT 0x40 /* only give out 32bit addresses */

+#define PROT_SHSTK 0x10 /* shadow stack pages */
+
#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
/*
* Take the 4 protection key bits out of the vma->vm_flags
@@ -19,13 +21,35 @@
((vm_flags) & VM_PKEY_BIT2 ? _PAGE_PKEY_BIT2 : 0) | \
((vm_flags) & VM_PKEY_BIT3 ? _PAGE_PKEY_BIT3 : 0))

-#define arch_calc_vm_prot_bits(prot, key) ( \
+#define pkey_vm_prot_bits(prot, key) ( \
((key) & 0x1 ? VM_PKEY_BIT0 : 0) | \
((key) & 0x2 ? VM_PKEY_BIT1 : 0) | \
((key) & 0x4 ? VM_PKEY_BIT2 : 0) | \
((key) & 0x8 ? VM_PKEY_BIT3 : 0))
+#else
+#define pkey_vm_prot_bits(prot, key)
#endif

+#define shstk_vm_prot_bits(prot) ( \
+ (static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK)) ? \
+ VM_SHSTK : 0)
+
+#define arch_calc_vm_prot_bits(prot, key) \
+ (pkey_vm_prot_bits(prot, key) | shstk_vm_prot_bits(prot))
+
#include <asm-generic/mman.h>

+static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
+{
+ unsigned long supported = PROT_READ | PROT_EXEC | PROT_SEM;
+
+ if (static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK))
+ supported |= PROT_SHSTK;
+ else
+ supported |= PROT_WRITE;
+
+ return (prot & ~supported) == 0;
+}
+#define arch_validate_prot arch_validate_prot
+
#endif /* _ASM_X86_MMAN_H */
diff --git a/mm/mprotect.c b/mm/mprotect.c
index a8edbcb3af99..520bd8caa005 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -571,6 +571,17 @@ static int do_mprotect_pkey(unsigned long start, size_t
len,
goto out;
}
}
+
+ /*
+ * Only anonymous mapping is suitable for shadow stack.
+ */
+ if (prot & PROT_SHSTK) {
+ if (vma->vm_file) {
+ error = -EINVAL;
+ goto out;
+ }
+ }
+
if (start > vma->vm_start)
prev = vma;

--