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_SHSTK 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.to...@gmail.com/ Signed-off-by: Yu-cheng Yu <yu-cheng...@intel.com> Reviewed-by: Kees Cook <keesc...@chromium.org> --- arch/x86/include/asm/mman.h | 57 +++++++++++++++++++++++++++++++- arch/x86/include/uapi/asm/mman.h | 1 + include/linux/mm.h | 1 + mm/mmap.c | 8 ++++- 4 files changed, 65 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h index 629f6c81263a..bd94e30b5d34 100644 --- a/arch/x86/include/asm/mman.h +++ b/arch/x86/include/asm/mman.h @@ -20,11 +20,66 @@ ((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_WRITE) && (prot & PROT_SHSTK)) + vm_prot_bits |= VM_SHSTK; + + return vm_prot_bits; +} + +#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey) + +#ifdef CONFIG_X86_CET +static inline bool arch_validate_prot(unsigned long prot, unsigned long addr) +{ + unsigned long valid = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM; + + if (prot & ~(valid | PROT_SHSTK)) + return false; + + if (prot & PROT_SHSTK) { + struct vm_area_struct *vma; + + 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; + + vma = find_vma(current->mm, addr); + if (!vma) + return false; + + /* + * Shadow stack cannot be backed by a file or shared. + */ + if (vma->vm_file || (vma->vm_flags & VM_SHARED)) + return false; + } + + return true; +} + +#define arch_validate_prot arch_validate_prot #endif #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 e178be052419..40c4b0fe7cc4 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_SHSTK #elif defined(CONFIG_PPC) # define VM_SAO VM_ARCH_1 /* Strong Access Ordering (powerpc) */ #elif defined(CONFIG_PARISC) diff --git a/mm/mmap.c b/mm/mmap.c index 99077171010b..934cb3cbe952 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1481,6 +1481,12 @@ unsigned long do_mmap(struct file *file, unsigned long addr, struct inode *inode = file_inode(file); unsigned long flags_mask; + /* + * Call stack cannot be backed by a file. + */ + if (vm_flags & VM_SHSTK) + return -EINVAL; + if (!file_mmap_ok(file, inode, pgoff, len)) return -EOVERFLOW; @@ -1545,7 +1551,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr, } else { switch (flags & MAP_TYPE) { case MAP_SHARED: - if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP)) + if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP|VM_SHSTK)) return -EINVAL; /* * Ignore pgoff. -- 2.21.0
