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

[Chen, Hu1]

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)
>  
>  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 <hu1.chen@xxxxxxxxx>
> > Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> > ---
> > 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/20221122073244.21279-1-hu1.chen@xxxxxxxxx/
> >
> > v1: https://lore.kernel.org/all/20221121085113.611504-1-hu1.chen@xxxxxxxxx/
> >
> >   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);