Re: [bpf-next v6 4/5] bpf, x86: Emit ENDBR for indirect jump targets

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2026-03-06 at 18:23 +0800, Xu Kuohai wrote:
> From: Xu Kuohai <xukuohai@huawei.com>
> 
> On CPUs that support CET/IBT, the indirect jump selftest triggers
> a kernel panic because the indirect jump targets lack ENDBR
> instructions.
> 
> To fix it, emit an ENDBR instruction to each indirect jump target. Since
> the ENDBR instruction shifts the position of original jited instructions,
> fix the instruction address calculation wherever the addresses are used.
> 
> For reference, below is a sample panic log.
> 
>  Missing ENDBR: bpf_prog_2e5f1c71c13ac3e0_big_jump_table+0x97/0xe1
>  ------------[ cut here ]------------
>  kernel BUG at arch/x86/kernel/cet.c:133!
>  Oops: invalid opcode: 0000 [#1] SMP NOPTI
> 
>  ...
> 
>   ? 0xffffffffc00fb258
>   ? bpf_prog_2e5f1c71c13ac3e0_big_jump_table+0x97/0xe1
>   bpf_prog_test_run_syscall+0x110/0x2f0
>   ? fdget+0xba/0xe0
>   __sys_bpf+0xe4b/0x2590
>   ? __kmalloc_node_track_caller_noprof+0x1c7/0x680
>   ? bpf_prog_test_run_syscall+0x215/0x2f0
>   __x64_sys_bpf+0x21/0x30
>   do_syscall_64+0x85/0x620
>   ? bpf_prog_test_run_syscall+0x1e2/0x2f0
> 
> Fixes: 493d9e0d6083 ("bpf, x86: add support for indirect jumps")
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 23 +++++++++++++++--------
>  1 file changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2c57ee446fc9..752331a64fc0 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1658,8 +1658,8 @@ static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,
>  	return 0;
>  }
>  
> -static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,
> -		  int oldproglen, struct jit_context *ctx, bool jmp_padding)
> +static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *addrs, u8 *image,
> +		  u8 *rw_image, int oldproglen, struct jit_context *ctx, bool jmp_padding)
>  {
>  	bool tail_call_reachable = bpf_prog->aux->tail_call_reachable;
>  	struct bpf_insn *insn = bpf_prog->insnsi;
> @@ -1743,6 +1743,11 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
>  				dst_reg = X86_REG_R9;
>  		}
>  
> +#ifdef CONFIG_X86_KERNEL_IBT
> +		if (bpf_insn_is_indirect_target(env, bpf_prog, i - 1))
> +			EMIT_ENDBR();
> +#endif
> +
>  		switch (insn->code) {
>  			/* ALU */
>  		case BPF_ALU | BPF_ADD | BPF_X:
> @@ -2449,7 +2454,7 @@ st:			if (is_imm8(insn->off))
>  
>  			/* call */
>  		case BPF_JMP | BPF_CALL: {
> -			u8 *ip = image + addrs[i - 1];
> +			u8 *ip = image + addrs[i - 1] + (prog - temp);

Sorry, meant to reply to v5 but got distracted.
It seems tedious/error prone to have this addend at each location,
would it be possible to move the 'ip' variable calculation outside
of the switch? It appears that at each point there would be no
EMIT invocations between 'ip' computation and usage.

>  
>  			func = (u8 *) __bpf_call_base + imm32;
>  			if (src_reg == BPF_PSEUDO_CALL && tail_call_reachable) {
> @@ -2474,7 +2479,8 @@ st:			if (is_imm8(insn->off))
>  			if (imm32)
>  				emit_bpf_tail_call_direct(bpf_prog,
>  							  &bpf_prog->aux->poke_tab[imm32 - 1],
> -							  &prog, image + addrs[i - 1],
> +							  &prog,
> +							  image + addrs[i - 1] + (prog - temp),
>  							  callee_regs_used,
>  							  stack_depth,
>  							  ctx);
> @@ -2483,7 +2489,7 @@ st:			if (is_imm8(insn->off))
>  							    &prog,
>  							    callee_regs_used,
>  							    stack_depth,
> -							    image + addrs[i - 1],
> +							    image + addrs[i - 1] + (prog - temp),
>  							    ctx);
>  			break;
>  
> @@ -2648,7 +2654,8 @@ st:			if (is_imm8(insn->off))
>  			break;
>  
>  		case BPF_JMP | BPF_JA | BPF_X:
> -			emit_indirect_jump(&prog, insn->dst_reg, image + addrs[i - 1]);
> +			emit_indirect_jump(&prog, insn->dst_reg,
> +					   image + addrs[i - 1] + (prog - temp));
>  			break;
>  		case BPF_JMP | BPF_JA:
>  		case BPF_JMP32 | BPF_JA:
> @@ -2738,7 +2745,7 @@ st:			if (is_imm8(insn->off))
>  			ctx->cleanup_addr = proglen;
>  			if (bpf_prog_was_classic(bpf_prog) &&
>  			    !ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN)) {
> -				u8 *ip = image + addrs[i - 1];
> +				u8 *ip = image + addrs[i - 1] + (prog - temp);
>  
>  				if (emit_spectre_bhb_barrier(&prog, ip, bpf_prog))
>  					return -EINVAL;
> @@ -3800,7 +3807,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
>  	for (pass = 0; pass < MAX_PASSES || image; pass++) {
>  		if (!padding && pass >= PADDING_PASSES)
>  			padding = true;
> -		proglen = do_jit(prog, addrs, image, rw_image, oldproglen, &ctx, padding);
> +		proglen = do_jit(env, prog, addrs, image, rw_image, oldproglen, &ctx, padding);
>  		if (proglen <= 0) {
>  out_image:
>  			image = NULL;