Subject: Re: [PATCH bpf v3] selftests/bpf: Fix "missing ENDBR" BUG for destructor kfunc

From: Jiri Olsa <[email-masked]>
Date: Thu, 1 Dec 2022 12:50:34 +0100

On Thu, Dec 01, 2022 at 04:07:56PM +0800, Chen, Hu1 wrote:
> On 12/1/2022 12:52 AM, Yonghong Song wrote:
> >  
> >  
> >  On 11/30/22 2:11 AM, Chen Hu wrote:
> > > With CONFIG_X86_KERNEL_IBT enabled, the test_verifier triggers the
> > > following BUG:
> > >
> > >    traps: Missing ENDBR: bpf_kfunc_call_test_release+0x0/0x30
> > >    ------------[ cut here ]------------
> > >    kernel BUG at arch/x86/kernel/traps.c:254!
> > >    invalid opcode: 0000 [#1] PREEMPT SMP
> > >    <TASK>
> > >     asm_exc_control_protection+0x26/0x50
> > >    RIP: 0010:bpf_kfunc_call_test_release+0x0/0x30
> > >    Code: 00 48 c7 c7 18 f2 e1 b4 e8 0d ca 8c ff 48 c7 c0 00 f2 e1 b4 c3
> > >     0f 1f 44 00 00 66 0f 1f 00 0f 1f 44 00 00 0f 0b 31 c0 c3 66 90
> > >         <66> 0f 1f 00 0f 1f 44 00 00 48 85 ff 74 13 4c 8d 47 18 b8 ff ff ff
> > >     bpf_map_free_kptrs+0x2e/0x70
> > >     array_map_free+0x57/0x140
> > >     process_one_work+0x194/0x3a0
> > >     worker_thread+0x54/0x3a0
> > >     ? rescuer_thread+0x390/0x390
> > >     kthread+0xe9/0x110
> > >     ? kthread_complete_and_exit+0x20/0x20
> > >
> > > It turns out that ENDBR in bpf_kfunc_call_test_release() is converted to
> > > NOP by apply_ibt_endbr().
> > >
> > > The only text references to this function from kernel side are:
> > >
> > >    $ grep -r bpf_kfunc_call_test_release
> > >    net/bpf/test_run.c:noinline void bpf_kfunc_call_test_release(...)
> > >    net/bpf/test_run.c:BTF_ID_FLAGS(func, bpf_kfunc_call_test_release, ...)
> > >    net/bpf/test_run.c:BTF_ID(func, bpf_kfunc_call_test_release)

Alexei mentioned we could now move these ^^^ into kernel module:
  https://lore.kernel.org/bpf/CAADnVQJ4xaAacOUpzMG+bm2WK5u=[email-masked]/

would it help? not sure objtool scans modules as well

but we'd still need fix for bpf_obj_new_impl/bpf_obj_drop_impl below

jirka

> >  
> >  We have some other function like this. For example, some newly added
> >  functions like bpf_obj_new_impl(), bpf_obj_drop_impl(), do they have
> >  the same missing endbr problem? If this is the case, we need a
> >  general solution.
> >
> 
> bpf_obj_new_impl(), bpf_obj_drop_impl() also miss the ENDBR. Below is
> the disassembly on bpf-next kernel:
> 
> (gdb) disas bpf_obj_drop_impl
> Dump of assembler code for function bpf_obj_drop_impl:
>    0xffffffff81288e40 <+0>:     nopw   (%rax)
>    0xffffffff81288e44 <+4>:     nopl   0x0(%rax,%rax,1)
>    0xffffffff81288e49 <+9>:     push   %rbp
>    ...
> 
> (gdb) disas bpf_obj_new_impl
> Dump of assembler code for function bpf_obj_new_impl:
>    0xffffffff81288cd0 <+0>:     nopw   (%rax)
>    0xffffffff81288cd4 <+4>:     nopl   0x0(%rax,%rax,1)
>    0xffffffff81288cd9 <+9>:     push   %rbp
>    ...
> 
> The first insn in the bpf_obj_new_impl has been converted from ENDBR to
> nopw by objtool. If the function is indirectly called on IBT enabled CPU
> (Tigerlake for example), #CP raise.
> 
> Looks like the possible fix in this patch is general?
> If we don't want to seal a funciton, we use macro IBT_NOSEAL to claim.
> IBT_NOSEAL just creates throwaway dummy compile-time references to the
> functions. The section is already thrown away when kernel run. See
> commit e27e5bea956c by Josh Poimboeuf.
> 
> > >
> > > but it may be called from bpf program as kfunc. (no other caller from
> > > kernel)
> > >
> > > This fix creates dummy references to destructor kfuncs so ENDBR stay
> > > there.
> > >
> > > Also modify macro XXX_NOSEAL slightly:
> > > - ASM_IBT_NOSEAL now stands for pure asm
> > > - IBT_NOSEAL can be used directly in C
> > >
> > > Signed-off-by: Chen Hu <[email-masked]>
> > > Tested-by: Pengfei Xu <[email-masked]>
> > > ---
> > > v3:
> > > - Macro go to IBT related header as suggested by Jiri Olsa
> > > - Describe reference to the func clearly in commit message as suggested
> > >    by Peter Zijlstra and Jiri Olsa
> > >   v2: https://lore.kernel.org/all/[email-masked]/
> > >
> > > v1: https://lore.kernel.org/all/[email-masked]/
> > >
> > >   arch/x86/include/asm/ibt.h | 6 +++++-
> > >   arch/x86/kvm/emulate.c     | 2 +-
> > >   net/bpf/test_run.c         | 5 +++++
> > >   3 files changed, 11 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h
> > > index 9b08082a5d9f..be86dc31661c 100644
> > > --- a/arch/x86/include/asm/ibt.h
> > > +++ b/arch/x86/include/asm/ibt.h
> > > @@ -36,11 +36,14 @@
> > >    * the function as needing to be "sealed" (i.e. ENDBR converted to NOP by
> > >    * apply_ibt_endbr()).
> > >    */
> > > -#define IBT_NOSEAL(fname)                \
> > > +#define ASM_IBT_NOSEAL(fname)                \
> > >       ".pushsection .discard.ibt_endbr_noseal\n\t"    \
> > >       _ASM_PTR fname "\n\t"                \
> > >       ".popsection\n\t"
> > >   +#define IBT_NOSEAL(name)                \
> > > +    asm(ASM_IBT_NOSEAL(#name))
> > > +
> > >   static inline __attribute_const__ u32 gen_endbr(void)
> > >   {
> > >       u32 endbr;
> > > @@ -94,6 +97,7 @@ extern __noendbr void ibt_restore(u64 save);
> > >   #ifndef __ASSEMBLY__
> > >     #define ASM_ENDBR
> > > +#define ASM_IBT_NOSEAL(name)
> > >   #define IBT_NOSEAL(name)
> > >     #define __noendbr
> > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > > index 4a43261d25a2..d870c8bb5831 100644
> > > --- a/arch/x86/kvm/emulate.c
> > > +++ b/arch/x86/kvm/emulate.c
> > > @@ -327,7 +327,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
> > >       ".type " name ", @function \n\t" \
> > >       name ":\n\t" \
> > >       ASM_ENDBR \
> > > -    IBT_NOSEAL(name)
> > > +    ASM_IBT_NOSEAL(name)
> > >     #define FOP_FUNC(name) \
> > >       __FOP_FUNC(#name)
> > > diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > > index fcb3e6c5e03c..9e9c8e8d50d7 100644
> > > --- a/net/bpf/test_run.c
> > > +++ b/net/bpf/test_run.c
> > > @@ -601,6 +601,11 @@ noinline void bpf_kfunc_call_memb_release(struct prog_test_member *p)
> > >   {
> > >   }
> > >   +#ifdef CONFIG_X86_KERNEL_IBT
> > > +IBT_NOSEAL(bpf_kfunc_call_test_release);
> > > +IBT_NOSEAL(bpf_kfunc_call_memb_release);
> > > +#endif
> > > +
> > >   noinline void bpf_kfunc_call_memb1_release(struct prog_test_member1 *p)
> > >   {
> > >       WARN_ON_ONCE(1);