[RFC bpf-next v4 3/6] bpf: Handle BPF_UPTR in verifier.

[Kui-Feng Lee <thinker.li@xxxxxxxxx>]

Give PTR_TO_MEM | PTR_MAYBE_NULL to the memory pointed by an uptr with the
size of the pointed type to make them readable and writable.

Signed-off-by: Kui-Feng Lee <thinker.li@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e3932f8ce10a..5bc5b37b63cc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5340,6 +5340,10 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
 	int perm_flags;
 	const char *reg_name = "";
 
+	if (kptr_field->type == BPF_UPTR)
+		/* BPF programs should not change any user kptr */
+		return -EACCES;
+
 	if (btf_is_kernel(reg->btf)) {
 		perm_flags = PTR_MAYBE_NULL | PTR_TRUSTED | MEM_RCU;
 
@@ -5488,6 +5492,29 @@ static u32 btf_ld_kptr_type(struct bpf_verifier_env *env, struct btf_field *kptr
 	return ret;
 }
 
+static int mark_uptr_ld_reg(struct bpf_verifier_env *env, u32 regno,
+			    struct btf_field *field)
+{
+	struct bpf_reg_state *val_reg;
+	const struct btf_type *t;
+	u32 type_id, tsz;
+
+	val_reg = reg_state(env, regno);
+	type_id = field->kptr.btf_id;
+	t = btf_type_id_size(field->kptr.btf, &type_id, &tsz);
+	if (!t) {
+		verbose(env, "The type of uptr is invalid");
+		return -EACCES;
+	}
+
+	mark_reg_known_zero(env, cur_regs(env), regno);
+	val_reg->type = PTR_TO_MEM | PTR_MAYBE_NULL;
+	val_reg->mem_size = tsz;
+	val_reg->id = ++env->id_gen;
+
+	return 0;
+}
+
 static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
 				 int value_regno, int insn_idx,
 				 struct btf_field *kptr_field)
@@ -5516,9 +5543,16 @@ static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
 		verbose(env, "store to referenced kptr disallowed\n");
 		return -EACCES;
 	}
+	if (class != BPF_LDX && kptr_field->type == BPF_UPTR) {
+		verbose(env, "store to uptr disallowed\n");
+		return -EACCES;
+	}
 
 	if (class == BPF_LDX) {
 		val_reg = reg_state(env, value_regno);
+		if (kptr_field->type == BPF_UPTR)
+			return mark_uptr_ld_reg(env, value_regno, kptr_field);
+
 		/* We can simply mark the value_regno receiving the pointer
 		 * value from map as PTR_TO_BTF_ID, with the correct type.
 		 */
@@ -5576,6 +5610,7 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
 			case BPF_KPTR_UNREF:
 			case BPF_KPTR_REF:
 			case BPF_KPTR_PERCPU:
+			case BPF_UPTR:
 				if (src != ACCESS_DIRECT) {
 					verbose(env, "kptr cannot be accessed indirectly by helper\n");
 					return -EACCES;
@@ -6956,7 +6991,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 			return err;
 		if (tnum_is_const(reg->var_off))
 			kptr_field = btf_record_find(reg->map_ptr->record,
-						     off + reg->var_off.value, BPF_KPTR);
+						     off + reg->var_off.value, BPF_KPTR | BPF_UPTR);
 		if (kptr_field) {
 			err = check_map_kptr_access(env, regno, value_regno, insn_idx, kptr_field);
 		} else if (t == BPF_READ && value_regno >= 0) {
-- 
2.34.1