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    SubjectRe: [PATCH] static_call,x86: Robustify trampoline patching
    On Tue, Nov 02, 2021 at 10:35:30AM -0700, Kees Cook wrote:
    > On Sat, Oct 30, 2021 at 10:16:31AM +0200, Peter Zijlstra wrote:
    > > foo.cfi:
    > > endbr
    > > xorl $0xdeadbeef, %r10d
    > > jz foo
    > > ud2
    > > nop # make it an even 16 bytes
    > > foo:
    > > # actual function text
    > >
    > >
    > > Then have the address of foo, be the address of foo, like any normal
    > > sane person would expect. Have direct calls to foo, go to foo, again, as
    > > expected.
    > >
    > > When doing an indirect call (to r11, as clang does), then, and only
    > > then, do:
    > >
    > > movl $0xdeadbeef, %r10d
    > > subq $0x10, %r11
    > > call *%r11
    > >
    > > # if the r11 lives, add:
    > > addq $0x10, %r11
    > >
    > >
    > > Then only when caller and callee agree 0xdeadbeef is the password, does
    > > the indirect call go through.
    > >
    > > Why isn't this a suitable CFI scheme even without IBT?
    >
    > The trouble is that the callee is doing the verification. There's no
    > protection against calling into a callee that doesn't perform a check
    > (e.g. BPF JIT, or otherwise constructed executable memory, etc). The
    > caller needs to do the verification that what they're calling into is
    > safe before it makes the call.

    Right, Ard said the same, see new crackpot scheme here:

    https://lkml.kernel.org/r/YYE1yPClPMHvyvIt@hirez.programming.kicks-ass.net

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