[PATCH bpf-next 7/8] bpf: Undo strict enforcement for walking untagged fields.

Alexei Starovoitov
2023-04-04T04:51:20Z

From: Alexei Starovoitov <ast@kernel.org>

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>
---
kernel/bpf/verifier.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index fd90ba498ccc..56f569811f70 100644
--- 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)
--
2.34.1