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Date:   Tue, 2 Nov 2021 19:14:25 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>, X86 ML <x86@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev, joao@overdrivepizza.com
Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching
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On Tue, Nov 02, 2021 at 06:44:56PM +0100, Ard Biesheuvel wrote:
> On Tue, 2 Nov 2021 at 16:15, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Nov 02, 2021 at 01:57:44PM +0100, Peter Zijlstra wrote:
> >
> > > So how insane is something like this, have each function:
> > >
> > > foo.cfi:
> > >       endbr64
> > >       xorl $0xdeadbeef, %r10d
> > >       jz foo
> > >       ud2
> > >       nop     # make it 16 bytes
> > > foo:
> > >       # actual function text goes here
> > >
> > >
> > > And for each hash have two thunks:
> > >
> > >
> > >       # arg: r11
> > >       # clobbers: r10, r11
> > > __x86_indirect_cfi_deadbeef:
> > >       movl -9(%r11), %r10             # immediate in foo.cfi
> > >       xorl $0xdeadbeef, %r10          # our immediate
> > >       jz 1f
> > >       ud2
> > > 1:    ALTERNATIVE_2   "jmp *%r11",
> > >                       "jmp __x86_indirect_thunk_r11", X86_FEATURE_RETPOLINE
> > >                       "lfence; jmp *%r11", X86_FEATURE_RETPOLINE_AMD
> > >
> 
> So are these supposed to go into the jump tables? If so, there still
> needs to be a check against the boundary of the table at the call
> site, to ensure that we are not calling something that we shouldn't.
> 
> If they are not going into the jump tables, I don't see the point of
> having them, as only happy flow/uncomprised code would bother to use
> them.

I don't understand. If you can scribble your own code, you can
circumvent pretty much any range check anyway. But if you can't scribble
your own code, you get to use the branch here and that checks the
callsite and callee signature.

The range check isn't fundamental to CFI, having a check is the
important thing AFAIU.