On Tue, Apr 27, 2021 at 01:43:15PM -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_SHADOW_STACK to mmap() and > 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_SHADOW_STACK to mmap()/mprotect() takes advantage > of existing APIs. The x86-specific PROT_SHADOW_STACK 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> > --- > v26: > - Change PROT_SHSTK to PROT_SHADOW_STACK. > - Remove (vm_flags & VM_SHARED) check, since it is covered by > !vma_is_anonymous(). > > 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 | 60 +++++++++++++++++++++++++++++++- > arch/x86/include/uapi/asm/mman.h | 2 ++ > include/linux/mm.h | 1 + > 3 files changed, 62 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h > index 629f6c81263a..fbb90f1b02c0 100644 > --- a/arch/x86/include/asm/mman.h > +++ b/arch/x86/include/asm/mman.h > @@ -20,11 +20,69 @@ > ((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) (0) > #endif > > +static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot, > + unsigned long pkey) > +{ > + unsigned long vm_prot_bits = pkey_vm_prot_bits(prot, pkey); > + > + if (prot & PROT_SHADOW_STACK) > + vm_prot_bits |= VM_SHADOW_STACK; > + > + return vm_prot_bits; > +} > + > +#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey) > + > +#ifdef CONFIG_X86_SHADOW_STACK > +static inline bool arch_validate_prot(unsigned long prot, unsigned long addr) > +{ > + unsigned long valid = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM | > + PROT_SHADOW_STACK; > + > + if (prot & ~valid) > + return false; > + > + if (prot & PROT_SHADOW_STACK) { > + if (!current->thread.cet.shstk_size) > + return false; > + > + /* > + * A shadow stack mapping is indirectly writable by only > + * the CALL and WRUSS instructions, but not other write > + * instructions). PROT_SHADOW_STACK and PROT_WRITE are > + * mutually exclusive. > + */ > + if (prot & PROT_WRITE) > + return false; > + } > + > + return true; > +} > + > +#define arch_validate_prot arch_validate_prot > + > +static inline bool arch_validate_flags(struct vm_area_struct *vma, unsigned long vm_flags) > +{ > + /* > + * Shadow stack must be anonymous and not shared. > + */ > + if ((vm_flags & VM_SHADOW_STACK) && !vma_is_anonymous(vma)) > + return false; > + > + return true; > +} > + > +#define arch_validate_flags(vma, vm_flags) arch_validate_flags(vma, vm_flags) > + > +#endif /* CONFIG_X86_SHADOW_STACK */ > + > #endif /* _ASM_X86_MMAN_H */ > diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h > index f28fa4acaeaf..4c36b263cf0a 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_SHADOW_STACK 0x10 /* shadow stack pages */ > + > #include <asm-generic/mman.h> > > #endif /* _UAPI_ASM_X86_MMAN_H */ > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 1ccec5cc399b..9a7652eea207 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -342,6 +342,7 @@ extern unsigned int kobjsize(const void *objp); > > #if defined(CONFIG_X86) > # define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */ > +# define VM_ARCH_CLEAR VM_SHADOW_STACK Nit: you can put VM_SHADOW_STACK directly into VM_FLAGS_CLEAR. It's already conditinal on the feature enabled and VM_NONE otherwise. Up to you. Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> -- Kirill A. Shutemov
