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

[Yonghong Song <yhs@meta.com>]

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.

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