| Message ID | 20230119212317.8324-17-rick.p.edgecombe@intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Shadow stacks for userspace | expand |
On Thu, Jan 19, 2023 at 01:22:54PM -0800, 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 stack accesses to shadow-stack mappings can result in 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. > > For the purpose of making this clearer, consider the following example. > If a process has a shadow stack, and forks, the shadow stack PTEs will > become read-only due to COW. If the CPU in one process performs a shadow > stack read access to the shadow stack, for example executing a RET and > causing the CPU to read the shadow stack copy of the return address, then > in order for the fault to be resolved the PTE will need to be set with > shadow stack permissions. But then the memory would be changeable from > userspace (from CALL, RET, WRSS, etc). So this scenario needs to trigger > COW, otherwise the shared page would be changeable from both processes. > > 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. > > Tested-by: Pengfei Xu <pengfei.xu@intel.com> > Tested-by: John Allen <john.allen@amd.com> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com> Reviewed-by: Kees Cook <keescook@chromium.org>
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 7b0d4ab894c8..070b50c87415 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -1138,8 +1138,22 @@ 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; @@ -1331,6 +1345,30 @@ void do_user_addr_fault(struct pt_regs *regs, perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address); + /* + * When a page becomes COW it changes from a shadow stack permission + * page (Write=0,Dirty=1) to (Write=0,Dirty=0,CoW=1), which is simply + * read-only to the CPU. When shadow stack is enabled, a RET would + * normally pop the shadow stack by reading it with a "shadow stack + * read" access. However, in the COW case the shadow stack memory does + * not have shadow stack permissions, it is read-only. So it will + * generate a fault. + * + * For conventionally writable pages, a read can be serviced with a + * read only PTE, and COW would not have to happen. But for shadow + * stack, there isn't the concept of read-only shadow stack memory. + * If it is shadow stack permission, it can be modified via CALL and + * RET instructions. So COW needs to happen before any memory can be + * mapped with shadow stack permissions. + * + * Shadow stack accesses (read or write) need to be serviced with + * shadow stack permission memory, so in the case of a shadow stack + * read access, treat it as a WRITE fault so both COW will happen and + * the write fault path will tickle maybe_mkwrite() and map the memory + * shadow stack. + */ + 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)