bpf: Refactor RCU enforcement in the verifier.

bpf_rcu_read_lock/unlock() are only available in clang compiled kernels. Lack
of such key mechanism makes it impossible for sleepable bpf programs to use RCU
pointers.

Allow bpf_rcu_read_lock/unlock() in GCC compiled kernels (though GCC doesn't
support btf_type_tag yet) and allowlist certain field dereferences in important
data structures like tast_struct, cgroup, socket that are used by sleepable
programs either as RCU pointer or full trusted pointer (which is valid outside
of RCU CS). Use BTF_TYPE_SAFE_RCU and BTF_TYPE_SAFE_TRUSTED macros for such
tagging. They will be removed once GCC supports btf_type_tag.

With that refactor check_ptr_to_btf_access(). Make it strict in enforcing
PTR_TRUSTED and PTR_UNTRUSTED while deprecating old PTR_TO_BTF_ID without
modifier flags. There is a chance that this strict enforcement might break
existing programs (especially on GCC compiled kernels), but this cleanup has to
start sooner than later. Note PTR_TO_CTX access still yields old deprecated
PTR_TO_BTF_ID. Once it's converted to strict PTR_TRUSTED or PTR_UNTRUSTED the
kfuncs and helpers will be able to default to KF_TRUSTED_ARGS. KF_RCU will
remain as a weaker version of KF_TRUSTED_ARGS where obj refcnt could be 0.

Adjust rcu_read_lock selftest to run on gcc and clang compiled kernels.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230303041446.3630-7-alexei.starovoitov@gmail.com

1 files changed, 13 insertions, 3 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index c5e1d6955491..a8cb09e5973b 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6163,6 +6163,7 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 	const char *tname, *mname, *tag_value;
 	u32 vlen, elem_id, mid;
 
+	*flag = 0;
 again:
 	tname = __btf_name_by_offset(btf, t->name_off);
 	if (!btf_type_is_struct(t)) {
@@ -6329,6 +6330,15 @@ error:
 		 * of this field or inside of this struct
 		 */
 		if (btf_type_is_struct(mtype)) {
+			if (BTF_INFO_KIND(mtype->info) == BTF_KIND_UNION &&
+			    btf_type_vlen(mtype) != 1)
+				/*
+				 * walking unions yields untrusted pointers
+				 * with exception of __bpf_md_ptr and other
+				 * unions with a single member
+				 */
+				*flag |= PTR_UNTRUSTED;
+
 			/* our field must be inside that union or struct */
 			t = mtype;
 
@@ -6373,7 +6383,7 @@ error:
 			stype = btf_type_skip_modifiers(btf, mtype->type, &id);
 			if (btf_type_is_struct(stype)) {
 				*next_btf_id = id;
-				*flag = tmp_flag;
+				*flag |= tmp_flag;
 				return WALK_PTR;
 			}
 		}
@@ -8357,7 +8367,7 @@ out:
 
 bool btf_nested_type_is_trusted(struct bpf_verifier_log *log,
 				const struct bpf_reg_state *reg,
-				int off)
+				int off, const char *suffix)
 {
 	struct btf *btf = reg->btf;
 	const struct btf_type *walk_type, *safe_type;
@@ -8374,7 +8384,7 @@ bool btf_nested_type_is_trusted(struct bpf_verifier_log *log,
 
 	tname = btf_name_by_offset(btf, walk_type->name_off);
 
-	ret = snprintf(safe_tname, sizeof(safe_tname), "%s__safe_fields", tname);
+	ret = snprintf(safe_tname, sizeof(safe_tname), "%s%s", tname, suffix);
 	if (ret < 0)
 		return false;