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    SubjectRe: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call
    On Mon, Sep 28, 2020 at 11:08 AM H.J. Lu <hjl.tools@gmail.com> wrote:
    >
    > On Mon, Sep 28, 2020 at 9:44 AM Andrew Cooper <andrew.cooper3@citrix.com> 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

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