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 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/[email protected]/ Signed-off-by: Yu-cheng Yu <[email protected]> Cc: Kees Cook <[email protected]> Cc: Kirill A. Shutemov <[email protected]> --- 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 @@ -20,11 +20,68 @@ ((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_SHSTK) + 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_SHSTK; + + if (prot & ~valid) + return false; + + if (prot & PROT_SHSTK) { + 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_SHSTK 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(unsigned long vm_flags, bool is_anon) +{ + if (vm_flags & VM_SHADOW_STACK) { + if ((vm_flags & VM_SHARED) || !is_anon) + return false; + } + + return true; +} + +#define arch_validate_flags(vm_flags, is_anon) arch_validate_flags(vm_flags, is_anon) + +#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 3ce1923e6ed9..39bb7db344a6 100644 --- a/arch/x86/include/uapi/asm/mman.h +++ b/arch/x86/include/uapi/asm/mman.h @@ -4,6 +4,7 @@ #define MAP_32BIT 0x40 /* only give out 32bit addresses */ +#define PROT_SHSTK 0x10 /* shadow stack pages */ #include <asm-generic/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 #elif defined(CONFIG_PPC) # define VM_SAO VM_ARCH_1 /* Strong Access Ordering (powerpc) */ #elif defined(CONFIG_PARISC) -- 2.21.0
