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

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

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;
 
--