bpf: Undo strict enforcement for walking untagged fields.

The commit 6fcd486b3a0a ("bpf: Refactor RCU enforcement in the verifier.")
broke several tracing bpf programs. Even in clang compiled kernels there are
many fields that are not marked with __rcu that are safe to read and pass into
helpers, but the verifier doesn't know that they're safe. Aggressively marking
them as PTR_UNTRUSTED was premature.

Fixes: 6fcd486b3a0a ("bpf: Refactor RCU enforcement in the verifier.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230404045029.82870-8-alexei.starovoitov@gmail.com

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index fd90ba4..56f5698 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4974,6 +4974,11 @@ static bool is_rcu_reg(const struct bpf_reg_state *reg)
        return reg->type & MEM_RCU;
 }
 
+static void clear_trusted_flags(enum bpf_type_flag *flag)
+{
+       *flag &= ~(BPF_REG_TRUSTED_MODIFIERS | MEM_RCU);
+}
+
 static int check_pkt_ptr_alignment(struct bpf_verifier_env *env,
                                   const struct bpf_reg_state *reg,
                                   int off, int size, bool strict)
@@ -5602,8 +5607,8 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
                        } else if (flag & (MEM_PERCPU | MEM_USER)) {
                                /* keep as-is */
                        } else {
-                               /* walking unknown pointers yields untrusted pointer */
-                               flag = PTR_UNTRUSTED;
+                               /* walking unknown pointers yields old deprecated PTR_TO_BTF_ID */
+                               clear_trusted_flags(&flag);
                        }
                } else {
                        /*
@@ -5617,7 +5622,7 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
                }
        } else {
                /* Old compat. Deprecated */
-               flag &= ~PTR_TRUSTED;
+               clear_trusted_flags(&flag);
        }
 
        if (atype == BPF_READ && value_regno >= 0)