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Date:   Wed, 22 Nov 2023 13:41:34 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     paul.walmsley@sifive.com, palmer@dabbelt.com,
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        mark.rutland@arm.com
Subject: Re: [PATCH 2/2] x86/cfi,bpf: Fix BPF JIT call
Message-ID: <20231122124134.GP4779@noisy.programming.kicks-ass.net>
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On Wed, Nov 22, 2023 at 12:15:17PM +0100, Peter Zijlstra wrote:

> Ah, so normally the __cfi_foo symbol would catch those, lemme see what I
> can do here.

I have the below delta (untested etc..), does that look about right?

---
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -845,19 +845,24 @@ enum cfi_mode cfi_mode __ro_after_init =
 #ifdef CONFIG_CFI_CLANG
 struct bpf_insn;
 
-extern unsigned int bpf_func_proto(const void *ctx,
-				   const struct bpf_insn *insn);
+/* Must match bpf_func_t / DEFINE_BPF_PROG_RUN() */
+extern unsigned int __bpf_prog_runX(const void *ctx,
+				    const struct bpf_insn *insn);
 
-__ADDRESSABLE(bpf_func_proto);
+/* 
+ * Force a reference to the external symbol so the compiler generates
+ * __kcfi_typid.
+ */
+__ADDRESSABLE(__bpf_prog_runX);
 
-/* u32 __ro_after_init cfi_bpf_hash = __kcfi_typeid_bpf_func_proto */
+/* u32 __ro_after_init cfi_bpf_hash = __kcfi_typeid___bpf_prog_runX */
 asm (
 "	.pushsection	.data..ro_after_init,\"aw\",@progbits	\n"
 "	.type	cfi_bpf_hash,@object				\n"
 "	.globl	cfi_bpf_hash					\n"
 "	.p2align	2, 0x0					\n"
 "cfi_bpf_hash:							\n"
-"	.long	__kcfi_typeid_bpf_func_proto			\n"
+"	.long	__kcfi_typeid___bpf_prog_runX			\n"
 "	.size	cfi_bpf_hash, 4					\n"
 "	.popsection						\n"
 );
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -308,15 +308,20 @@ static void pop_callee_regs(u8 **pprog,
 	*pprog = prog;
 }
 
+/*
+ * Emit the various CFI preambles, see the large comment about FineIBT
+ * in arch/x86/kernel/alternative.c
+ */
+
 static int emit_fineibt(u8 **pprog)
 {
 	u8 *prog = *pprog;
 
 	EMIT_ENDBR();
-	EMIT3_off32(0x41, 0x81, 0xea, cfi_bpf_hash);
-	EMIT2(0x74, 0x07);
-	EMIT2(0x0f, 0x0b);
-	EMIT1(0x90);
+	EMIT3_off32(0x41, 0x81, 0xea, cfi_bpf_hash);	/* subl $hash, %r10d	*/
+	EMIT2(0x74, 0x07);				/* jz.d8 +7		*/
+	EMIT2(0x0f, 0x0b);				/* ud2			*/
+	EMIT1(0x90);					/* nop			*/
 	EMIT_ENDBR_POISON();
 
 	*pprog = prog;
@@ -328,7 +333,7 @@ static int emit_kcfi(u8 **pprog)
 	u8 *prog = *pprog;
 	int offset = 5;
 
-	EMIT1_off32(0xb8, cfi_bpf_hash);
+	EMIT1_off32(0xb8, cfi_bpf_hash);		/* movl $hash, %eax	*/
 #ifdef CONFIG_CALL_PADDING
 	EMIT1(0x90);
 	EMIT1(0x90);
@@ -3009,6 +3014,10 @@ struct bpf_prog *bpf_int_jit_compile(str
 			jit_data->header = header;
 			jit_data->rw_header = rw_header;
 		}
+		/*
+		 * ctx.prog_offset is used when CFI preambles put code *before*
+		 * the function. See emit_cfi().
+		 */
 		prog->bpf_func = (void *)image + ctx.prog_offset;
 		prog->jited = 1;
 		prog->jited_len = proglen - ctx.prog_offset; // XXX?
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1431,6 +1431,9 @@ struct bpf_prog_aux {
 	struct bpf_kfunc_desc_tab *kfunc_tab;
 	struct bpf_kfunc_btf_tab *kfunc_btf_tab;
 	u32 size_poke_tab;
+#ifdef CONFIG_FINEIBT
+	struct bpf_ksym ksym_prefix;
+#endif
 	struct bpf_ksym ksym;
 	const struct bpf_prog_ops *ops;
 	struct bpf_map **used_maps;
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -683,6 +683,23 @@ void bpf_prog_kallsyms_add(struct bpf_pr
 	fp->aux->ksym.prog = true;
 
 	bpf_ksym_add(&fp->aux->ksym);
+
+#ifdef CONFIG_FINEIBT
+	/*
+	 * When FineIBT, code in the __cfi_foo() symbols can get executed
+	 * and hence unwinder needs help.
+	 */
+	if (cfi_mode != CFI_FINEIBT)
+		return;
+
+	snprintf(fp->aux->ksym_prefix.name, KSYM_NAME_LEN,
+		 "__cfi_%s", fp->aux->ksym.name);
+
+	prog->aux->ksym_prefix.start = (unsigned long) prog->bpf_func - 16;
+	prog->aux->ksym_prefix.end   = (unsigned long) prog->bpf_func;
+
+	bpf_ksym_add(&fp->aux->ksym_prefix);
+#endif
 }
 
 void bpf_prog_kallsyms_del(struct bpf_prog *fp)