[RFC PATCH bpf 1/6] bpf: Disallow interpreter fallback for user BPF_ADDR_SPACE_CAST insn

[Leon Hwang <leon.hwang@linux.dev>]

The interpreter is unable to handle the user BPF_ADDR_SPACE_CAST insn,
whose '->off' is 1:

static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
{
	ALU64_MOV_X:
		switch (OFF) {
		case 0:
			DST = SRC;
			break;
		case 8:
			DST = (s8) SRC;
			break;
		case 16:
			DST = (s16) SRC;
			break;
		case 32:
			DST = (s32) SRC;
			break;
		}
		CONT;
}

On the fallback path from JIT in __bpf_prog_select_runtime(), reject
the insn to avoid being ignored by interpreter.

Fixes: 142fd4d2dcf5 ("bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/core.c | 29 +++++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 0db6e55bad52..e92eb8b7f945 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2608,23 +2608,37 @@ static struct bpf_prog *bpf_prog_jit_compile(struct bpf_verifier_env *env, struc
 	return prog;
 }
 
+static bool bpf_insn_requires_jit(struct bpf_insn *insn)
+{
+	if (insn_is_cast_user(insn))
+		return true;
+
+	return false;
+}
+
 /* Fix up helper call offsets on JIT fallback path. */
-static void bpf_fixup_fallback_helpers(struct bpf_verifier_env *env, struct bpf_prog *fp)
+static int bpf_fixup_fallback_helpers(struct bpf_verifier_env *env, struct bpf_prog *fp)
 {
 	struct bpf_insn *insn = fp->insnsi;
 	const struct bpf_func_proto *fn;
 	int i;
 
-	if (!env || !env->ops->get_func_proto)
-		return;
+	if (!env)
+		return 0;
 
 	for (i = 0; i < fp->len; i++, insn++) {
-		if (bpf_helper_call(insn) && bpf_jit_inlines_helper_call(insn->imm)) {
+		if (env->ops->get_func_proto && bpf_helper_call(insn) &&
+		    bpf_jit_inlines_helper_call(insn->imm)) {
 			fn = env->ops->get_func_proto(insn->imm, env->prog);
 			if (fn && fn->func)
 				insn->imm = fn->func - __bpf_call_base;
 		}
+
+		if (bpf_insn_requires_jit(insn))
+			return -EOPNOTSUPP;
 	}
+
+	return 0;
 }
 
 struct bpf_prog *__bpf_prog_select_runtime(struct bpf_verifier_env *env, struct bpf_prog *fp,
@@ -2663,8 +2677,11 @@ struct bpf_prog *__bpf_prog_select_runtime(struct bpf_verifier_env *env, struct
 			return fp;
 		}
 
-		if (!fp->jited)
-			bpf_fixup_fallback_helpers(env, fp);
+		if (!fp->jited) {
+			*err = bpf_fixup_fallback_helpers(env, fp);
+			if (*err)
+				return fp;
+		}
 	} else {
 		*err = bpf_prog_offload_compile(fp);
 		if (*err)
-- 
2.54.0