Merge branch 'bpf: Allow helpers access ptr_to_btf_id.'

Alexei Starovoitov says:

====================

From: Alexei Starovoitov <ast@kernel.org>

Allow code like:
bpf_strncmp(task->comm, 16, "foo");
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/helpers.c

@@ -571,7 +571,7 @@ static const struct bpf_func_proto bpf_strncmp_proto = {
 	.func		= bpf_strncmp,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_MEM,
+	.arg1_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
 	.arg2_type	= ARG_CONST_SIZE,
 	.arg3_type	= ARG_PTR_TO_CONST_STR,
 };

kernel/bpf/verifier.c

@@ -6303,6 +6303,9 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
 				env,
 				regno, reg->off, access_size,
 				zero_size_allowed, ACCESS_HELPER, meta);
+	case PTR_TO_BTF_ID:
+		return check_ptr_to_btf_access(env, regs, regno, reg->off,
+					       access_size, BPF_READ, -1);
 	case PTR_TO_CTX:
 		/* in case the function doesn't know how to access the context,
 		 * (because we are in a program of type SYSCALL for example), we
@@ -7014,6 +7017,7 @@ static const struct bpf_reg_types mem_types = {
 		PTR_TO_MEM,
 		PTR_TO_MEM | MEM_RINGBUF,
 		PTR_TO_BUF,
+		PTR_TO_BTF_ID | PTR_TRUSTED,
 	},
 };
 
@@ -7145,6 +7149,17 @@ found:
 	if (base_type(reg->type) != PTR_TO_BTF_ID)
 		return 0;
 
+	if (compatible == &mem_types) {
+		if (!(arg_type & MEM_RDONLY)) {
+			verbose(env,
+				"%s() may write into memory pointed by R%d type=%s\n",
+				func_id_name(meta->func_id),
+				regno, reg_type_str(env, reg->type));
+			return -EACCES;
+		}
+		return 0;
+	}
+
 	switch ((int)reg->type) {
 	case PTR_TO_BTF_ID:
 	case PTR_TO_BTF_ID | PTR_TRUSTED:

tools/testing/selftests/bpf/progs/task_kfunc_failure.c

@@ -301,3 +301,39 @@ int BPF_PROG(task_kfunc_from_lsm_task_free, struct task_struct *task)
 	bpf_task_release(acquired);
 	return 0;
 }
+
+SEC("tp_btf/task_newtask")
+__failure __msg("access beyond the end of member comm")
+int BPF_PROG(task_access_comm1, struct task_struct *task, u64 clone_flags)
+{
+	bpf_strncmp(task->comm, 17, "foo");
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+__failure __msg("access beyond the end of member comm")
+int BPF_PROG(task_access_comm2, struct task_struct *task, u64 clone_flags)
+{
+	bpf_strncmp(task->comm + 1, 16, "foo");
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+__failure __msg("write into memory")
+int BPF_PROG(task_access_comm3, struct task_struct *task, u64 clone_flags)
+{
+	bpf_probe_read_kernel(task->comm, 16, task->comm);
+	return 0;
+}
+
+SEC("fentry/__set_task_comm")
+__failure __msg("R1 type=ptr_ expected")
+int BPF_PROG(task_access_comm4, struct task_struct *task, const char *buf, bool exec)
+{
+	/*
+	 * task->comm is a legacy ptr_to_btf_id. The verifier cannot guarantee
+	 * its safety. Hence it cannot be accessed with normal load insns.
+	 */
+	bpf_strncmp(task->comm, 16, "foo");
+	return 0;
+}

tools/testing/selftests/bpf/progs/task_kfunc_success.c

@@ -207,6 +207,10 @@ int BPF_PROG(test_task_from_pid_invalid, struct task_struct *task, u64 clone_fla
 	if (!is_test_kfunc_task())
 		return 0;
 
+	bpf_strncmp(task->comm, 12, "foo");
+	bpf_strncmp(task->comm, 16, "foo");
+	bpf_strncmp(&task->comm[8], 4, "foo");
+
 	if (is_pid_lookup_valid(-1)) {
 		err = 1;
 		return 0;