On 4/25/2021 11:52 PM, Kirill A. Shutemov wrote:

On Thu, Apr 15, 2021 at 03:14:19PM -0700, Yu-cheng Yu wrote:

quoted

There are three possible options to create a shadow stack allocation API:
an arch_prctl, a new syscall, or adding PROT_SHSTK to mmap()/mprotect().
Each has its advantages and compromises.

An arch_prctl() is the least intrusive.  However, the existing x86
arch_prctl() takes only two parameters.  Multiple parameters must be
passed in a memory buffer.  There is a proposal to pass more parameters in
registers [1], but no active discussion on that.

A new syscall minimizes compatibility issues and offers an extensible frame
work to other architectures, but this will likely result in some overlap of
mmap()/mprotect().

The introduction of PROT_SHSTK to mmap()/mprotect() takes advantage of

Maybe PROT_SHADOW_STACK?

quoted

existing APIs.  The x86-specific PROT_SHSTK is translated to
VM_SHADOW_STACK and a shadow stack mapping is created without reinventing
the wheel.  There are potential pitfalls though.  The most obvious one
would be using this as a bypass to shadow stack protection.  However, the
attacker would have to get to the syscall first.

[1] https://lore.kernel.org/lkml/20200828121624.108243-1-hjl.tools@gmail.com/ (local)

Signed-off-by: Yu-cheng Yu <redacted>
Cc: Kees Cook <redacted>
Cc: Kirill A. Shutemov <redacted>
---
v24:
- Update arch_calc_vm_prot_bits(), leave PROT* checking to
   arch_validate_prot().
- Update arch_validate_prot(), leave vma flags checking to
   arch_validate_flags().
- Add arch_validate_flags().

  arch/x86/include/asm/mman.h      | 59 +++++++++++++++++++++++++++++++-
  arch/x86/include/uapi/asm/mman.h |  1 +
  include/linux/mm.h               |  1 +
  3 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h
index 629f6c81263a..1821c179f35d 100644
--- a/arch/x86/include/asm/mman.h
+++ b/arch/x86/include/asm/mman.h

[...]

quoted

+
+#define arch_validate_prot arch_validate_prot
+
+static inline bool arch_validate_flags(unsigned long vm_flags, bool is_anon)
+{
+	if (vm_flags & VM_SHADOW_STACK) {
+		if ((vm_flags & VM_SHARED) || !is_anon)

VM_SHARED check is redundant. vma_is_anonymous() should be enough.
Anonymous shared mappings would fail vma_is_anonymous().

Thanks for looking into this.  I will update and send another version.

Yu-cheng