linux-kernel - [PATCH] bpf: Refactor ptr alu checking rules to allow alu explicitly

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Message-ID: <20240117094012.36798-1-sunhao.th@gmail.com>
Date: Wed, 17 Jan 2024 10:40:12 +0100
From: Hao Sun <sunhao.th@...il.com>
To: bpf@...r.kernel.org
Cc: ast@...nel.org,
	andrii@...nel.org,
	daniel@...earbox.net,
	eddyz87@...il.com,
	linux-kernel@...r.kernel.org,
	Hao Sun <sunhao.th@...il.com>
Subject: [PATCH] bpf: Refactor ptr alu checking rules to allow alu explicitly

Current checking rules are structured to disallow alu on particular ptr
types explicitly, so default cases are allowed implicitly. This may lead
to newly added ptr types being allowed unexpectedly. So restruture it to
allow alu explicitly. The tradeoff is mainly a bit more cases added in
the switch. The following table from Eduard summarizes the rules:

        | Pointer type        | Arithmetics allowed |
        |---------------------+---------------------|
        | PTR_TO_CTX          | yes                 |
        | CONST_PTR_TO_MAP    | conditionally       |
        | PTR_TO_MAP_VALUE    | yes                 |
        | PTR_TO_MAP_KEY      | yes                 |
        | PTR_TO_STACK        | yes                 |
        | PTR_TO_PACKET_META  | yes                 |
        | PTR_TO_PACKET       | yes                 |
        | PTR_TO_PACKET_END   | no                  |
        | PTR_TO_FLOW_KEYS    | conditionally       |
        | PTR_TO_SOCKET       | no                  |
        | PTR_TO_SOCK_COMMON  | no                  |
        | PTR_TO_TCP_SOCK     | no                  |
        | PTR_TO_TP_BUFFER    | yes                 |
        | PTR_TO_XDP_SOCK     | no                  |
        | PTR_TO_BTF_ID       | yes                 |
        | PTR_TO_MEM          | yes                 |
        | PTR_TO_BUF          | yes                 |
        | PTR_TO_FUNC         | yes                 |
        | CONST_PTR_TO_DYNPTR | yes                 |

The refactored rules are equivalent to the original one. Note that
PTR_TO_FUNC and CONST_PTR_TO_DYNPTR are not reject here because: (1)
check_mem_access() rejects load/store on those ptrs, and those ptrs
with offset passing to calls are rejected check_func_arg_reg_off();
(2) someone may rely on the verifier not rejecting programs earily.

Signed-off-by: Hao Sun <sunhao.th@...il.com>
---
 kernel/bpf/verifier.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 65f598694d55..861d8d824bb8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12826,6 +12826,19 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	}
 
 	switch (base_type(ptr_reg->type)) {
+	case PTR_TO_CTX:
+	case PTR_TO_MAP_VALUE:
+	case PTR_TO_MAP_KEY:
+	case PTR_TO_STACK:
+	case PTR_TO_PACKET_META:
+	case PTR_TO_PACKET:
+	case PTR_TO_TP_BUFFER:
+	case PTR_TO_BTF_ID:
+	case PTR_TO_MEM:
+	case PTR_TO_BUF:
+	case PTR_TO_FUNC:
+	case CONST_PTR_TO_DYNPTR:
+		break;
 	case PTR_TO_FLOW_KEYS:
 		if (known)
 			break;
@@ -12835,16 +12848,10 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 		if (known && smin_val == 0 && opcode == BPF_ADD)
 			break;
 		fallthrough;
-	case PTR_TO_PACKET_END:
-	case PTR_TO_SOCKET:
-	case PTR_TO_SOCK_COMMON:
-	case PTR_TO_TCP_SOCK:
-	case PTR_TO_XDP_SOCK:
+	default:
 		verbose(env, "R%d pointer arithmetic on %s prohibited\n",
 			dst, reg_type_str(env, ptr_reg->type));
 		return -EACCES;
-	default:
-		break;
 	}
 
 	/* In case of 'scalar += pointer', dst_reg inherits pointer type and id.
-- 
2.34.1