| Message ID | 20220929222936.14584-16-rick.p.edgecombe@intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Shadowstacks for userspace | expand |
On Thu, Sep 29, 2022 at 03:29:12PM -0700, Rick Edgecombe wrote: > From: Yu-cheng Yu <yu-cheng.yu@intel.com> > > 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@intel.com> Reviewed-by: Kees Cook <keescook@chromium.org>
On Thu, Sep 29, 2022 at 03:29:12PM -0700, Rick Edgecombe wrote: > 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. That ^ should be moved into the comment below > - Clarify reasoning for FAULT_FLAG_WRITE for all shadow stack accesses > @@ -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. > + */ Because that's impenetrable. > + 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 >
On Fri, 2022-10-14 at 12:07 +0200, Peter Zijlstra wrote:
> That ^ should be moved into the comment below
Ok, good idea. Thanks.
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)