Re: [PATCH bpf-next v9 4/5] bpf, x86: Emit ENDBR for indirect jump targets

From: Xu Kuohai <redacted>

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")
Reviewed-by: Anton Protopopov <redacted>
Signed-off-by: Xu Kuohai <redacted>
---
arch/x86/net/bpf_jit_comp.c | 26 +++++++++++++++-----------
1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 72d9a5faa230..2d29830700f1 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c

@@ -1649,8 +1649,8 @@ 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;

@@ -1663,7 +1663,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image

	void __percpu *priv_stack_ptr;
	int i, excnt = 0;
	int ilen, proglen = 0;
-	u8 *prog = temp;
+	u8 *ip, *prog = temp;
	u32 stack_depth;
	int err;

@@ -1734,6 +1734,13 @@ 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

+
+		ip = image + addrs[i - 1] + (prog - temp);
+
		switch (insn->code) {
			/* ALU */
		case BPF_ALU | BPF_ADD | BPF_X:

@@ -2440,8 +2447,6 @@ st:			if (is_imm8(insn->off))

			/* call */
		case BPF_JMP | BPF_CALL: {
-			u8 *ip = image + addrs[i - 1];
-
			func = (u8 *) __bpf_call_base + imm32;
			if (src_reg == BPF_PSEUDO_CALL && tail_call_reachable) {
				LOAD_TAIL_CALL_CNT_PTR(stack_depth);

@@ -2465,7 +2470,8 @@ 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,
+							  ip,
							  callee_regs_used,
							  stack_depth,
							  ctx);

@@ -2474,7 +2480,7 @@ st:			if (is_imm8(insn->off))

							    &prog,
							    callee_regs_used,
							    stack_depth,
-							    image + addrs[i - 1],
+							    ip,
							    ctx);
			break;

@@ -2639,7 +2645,7 @@ 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, ip);
			break;
		case BPF_JMP | BPF_JA:
		case BPF_JMP32 | BPF_JA:

@@ -2729,8 +2735,6 @@ 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];
-
				if (emit_spectre_bhb_barrier(&prog, ip, bpf_prog))
					return -EINVAL;
			}

@@ -3791,7 +3795,7 @@ 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;