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Date:   Wed, 16 Mar 2022 10:35:38 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        X86 ML <x86@kernel.org>, joao@overdrivepizza.com,
        hjl.tools@gmail.com, Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>, alyssa.milburn@intel.com,
        Miroslav Benes <mbenes@suse.cz>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH v4 00/45] x86: Kernel IBT
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On Tue, Mar 15, 2022 at 10:00:43AM +0100, Peter Zijlstra wrote:
> On Tue, Mar 15, 2022 at 02:14:02AM +0530, Kumar Kartikeya Dwivedi wrote:
> 
> > [ Note: I have no experience with trampoline code or IBT so what follows might
> > 	be incorrect. ]
> > 
> > In case of fexit and fmod_ret, we call original function (but skip
> > X86_PATCH_SIZE bytes), with ENDBR we must also skip those 4 bytes, but in some
> > cases like bpf_fentry_test1, for which this test has fmod_ret prog, compiler
> > (gcc 11) emits endbr64, but not for do_init_module, for which we do fexit.
> > 
> > This means for do_init_module module, orig_call += X86_PATCH_SIZE +
> > ENDBR_INSN_SIZE would skip more bytes than needed to emit call to original
> > function, which explains why I was seeing crash in the middle of
> > 'mov edx, 0x10' instruction.
> > 
> > The diff below fixes the problem for me, and allows the test to pass.
> > 
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index b98e1c95bcc4..760c9a3c075f 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -2031,11 +2031,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > 
> >         ip_off = stack_size;
> > 
> > -       if (flags & BPF_TRAMP_F_SKIP_FRAME)
> > +       if (flags & BPF_TRAMP_F_SKIP_FRAME) {
> >                 /* skip patched call instruction and point orig_call to actual
> >                  * body of the kernel function.
> >                  */
> > -               orig_call += X86_PATCH_SIZE + ENDBR_INSN_SIZE;
> > +               if (is_endbr(*(u32 *)orig_call))
> > +                       orig_call += ENDBR_INSN_SIZE;
> > +               orig_call += X86_PATCH_SIZE;
> > +       }
> > 
> >         prog = image;
> 
> Hmm, so I was under the impression that this was targeting the NOP from
> emit_prologue(), and that has an unconditional ENDBR. If this is instead
> targeting the 'start of random kernel function' then yes, what you
> propose will work.

Can you confirm that orig_call can be any kernel function? Because if
so, I'm thinking it will still do the wrong thing for a notrace
function, that will not have a __fentry__ site, so unconditionally
skipping those 5 bytes will place you somewhere non-sensible.

This would not be a new issue; but perhaps it should be clarified and or
fixed.