From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> The CPU performs "shadow stack accesses" when it expects to encounter shadow stack mappings. These accesses can be implicit (via CALL/RET instructions) or explicit (instructions like WRSS). Shadow stacks accesses to shadow-stack mappings can see faults in normal, valid operation just like regular accesses to regular mappings. Shadow stacks need some of the same features like delayed allocation, swap and copy-on-write. The kernel needs to use faults to implement those features. The architecture has concepts of both shadow stack reads and shadow stack writes. Any shadow stack access to non-shadow stack memory will generate a fault with the shadow stack error code bit set. This means that, unlike normal write protection, the fault handler needs to create a type of memory that can be written to (with instructions that generate shadow stack writes), even to fulfill a read access. So in the case of COW memory, the COW needs to take place even with a shadow stack read. Otherwise the page will be left (shadow stack) writable in userspace. So to trigger the appropriate behavior, set FAULT_FLAG_WRITE for shadow stack accesses, even if the access was a shadow stack read. Shadow stack accesses can also result in errors, such as when a shadow stack overflows, or if a shadow stack access occurs to a non-shadow-stack mapping. Also, generate the errors for invalid shadow stack accesses. Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> --- v2: - Update commit log with verbiage/feedback from Dave Hansen - Clarify reasoning for FAULT_FLAG_WRITE for all shadow stack accesses - Update comments with some verbiage from Dave Hansen Yu-cheng v30: - Update Subject line and add a verb arch/x86/include/asm/trap_pf.h | 2 ++ arch/x86/mm/fault.c | 21 +++++++++++++++++++++ 2 files changed, 23 insertions(+) diff --git a/arch/x86/include/asm/trap_pf.h b/arch/x86/include/asm/trap_pf.h index 10b1de500ab1..afa524325e55 100644 --- a/arch/x86/include/asm/trap_pf.h +++ b/arch/x86/include/asm/trap_pf.h @@ -11,6 +11,7 @@ * bit 3 == 1: use of reserved bit detected * bit 4 == 1: fault was an instruction fetch * bit 5 == 1: protection keys block access + * bit 6 == 1: shadow stack access fault * bit 15 == 1: SGX MMU page-fault */ enum x86_pf_error_code { @@ -20,6 +21,7 @@ enum x86_pf_error_code { X86_PF_RSVD = 1 << 3, X86_PF_INSTR = 1 << 4, X86_PF_PK = 1 << 5, + X86_PF_SHSTK = 1 << 6, X86_PF_SGX = 1 << 15, }; diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index fa71a5d12e87..e5697b393069 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -1107,8 +1107,22 @@ access_error(unsigned long error_code, struct vm_area_struct *vma) (error_code & X86_PF_INSTR), foreign)) return 1; + /* + * Shadow stack accesses (PF_SHSTK=1) are only permitted to + * shadow stack VMAs. All other accesses result in an error. + */ + if (error_code & X86_PF_SHSTK) { + if (unlikely(!(vma->vm_flags & VM_SHADOW_STACK))) + return 1; + if (unlikely(!(vma->vm_flags & VM_WRITE))) + return 1; + return 0; + } + if (error_code & X86_PF_WRITE) { /* write, present and write, not present: */ + if (unlikely(vma->vm_flags & VM_SHADOW_STACK)) + return 1; if (unlikely(!(vma->vm_flags & VM_WRITE))) return 1; return 0; @@ -1300,6 +1314,13 @@ void do_user_addr_fault(struct pt_regs *regs, perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address); + /* + * In order to fullfull a shadow stack access, the page needs + * to be made (shadow stack) writable. So treat all shadow stack + * accesses as writes. + */ + if (error_code & X86_PF_SHSTK) + flags |= FAULT_FLAG_WRITE; if (error_code & X86_PF_WRITE) flags |= FAULT_FLAG_WRITE; if (error_code & X86_PF_INSTR) -- 2.17.1