Re: [PATCH v25 30/30] mm: Introduce PROT_SHSTK for shadow stack

[Yu, Yu-cheng]

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:
> > 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?
>
> > 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@xxxxxxxxx/
> >
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> > 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

[...]

> > +
> > +#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