On Mon, Sep 28, 2020 at 2:56 PM Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote: > > On Mon, Sep 28, 2020 at 11:12:08AM -0700, Andy Lutomirski wrote: > > On Mon, Sep 28, 2020 at 11:08 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: > > > > > > On Mon, Sep 28, 2020 at 9:44 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote: > > > > > > > > On 28/09/2020 01:58, Jarkko Sakkinen wrote: > > > > > On Fri, Sep 25, 2020 at 07:23:59PM +0100, Andrew Cooper wrote: > > > > >> On 15/09/2020 12:28, Jarkko Sakkinen wrote: > > > > >>> diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S > > > > >>> new file mode 100644 > > > > >>> index 000000000000..adbd59d41517 > > > > >>> --- /dev/null > > > > >>> +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S > > > > >>> @@ -0,0 +1,157 @@ > > > > >>> +SYM_FUNC_START(__vdso_sgx_enter_enclave) > > > > >>> <snip> > > > > >>> +.Lretpoline: > > > > >>> + call 2f > > > > >>> +1: pause > > > > >>> + lfence > > > > >>> + jmp 1b > > > > >>> +2: mov %rax, (%rsp) > > > > >>> + ret > > > > >> I hate to throw further spanners in the work, but this is not compatible > > > > >> with CET, and the user shadow stack work in progress. > > > > > CET goes beyond my expertise. Can you describe, at least rudimentary, > > > > > how this code is not compatible? > > > > > > > > CET Shadow Stacks detect attacks which modify the return address on the > > > > stack. > > > > > > > > Retpoline *is* a ROP gadget. It really does modify the return address > > > > on the stack, even if its purpose is defensive (vs Spectre v2) rather > > > > than malicious. > > > > > > > > >> Whichever of these two large series lands first is going to inflict > > > > >> fixing this problem on the other. > > > > >> > > > > >> As the vdso text is global (to a first approximation), it must not be a > > > > >> retpoline if any other process is liable to want to use CET-SS. > > > > > Why is that? > > > > > > > > Because when CET-SS is enabled, the ret will suffer a #CP exception > > > > (return address on the stack not matching the one recorded in the shadow > > > > stack), which I presume/hope is wired into SIGSEGV. > > > > > > > > > > Here is the CET compatible retpoline: > > > > > > endbr64 > > > /* Check if shadow stack is in use. NB: R11 is the only usable > > > scratch register for function calls. */ > > > xorl %r11d, %r11d > > > rdsspq %r11 > > > testq %r11, %r11 > > > jnz 3f > > > call 2f > > > 1: > > > pause > > > lfence > > > jmp 1b > > > 2: > > > mov %rax, (%rsp) > > > ret > > > 3: > > > /* Shadow stack is in use. Make the indirect call. */ > > > call *%rax > > > ret > > > > What do we expect user programs to do on CET systems? It would be > > nice if we could instead ALTERNATIVE this out if X86_FEATURE_SHSTK. > > > > --Andy > > I'm open to do either solution. My thinking was to initially do things > vsgx.S local (i.e. consider ALTERNATIVE post upstreaming) and use the > above solution but I'm also fine doing ALTERNATIVE. Dave kindly briefed > on details how that thing works and it should be perfectly usable for > our use case. > Since SHSTK and IBT are enabled per process, not the whole machine, are you going to patch vDSO on a per-process basis? -- H.J.
