bpf: reject program if a __user tagged memory accessed in kernel way

BPF verifier supports direct memory access for BPF_PROG_TYPE_TRACING type
of bpf programs, e.g., a->b. If "a" is a pointer
pointing to kernel memory, bpf verifier will allow user to write
code in C like a->b and the verifier will translate it to a kernel
load properly. If "a" is a pointer to user memory, it is expected
that bpf developer should be bpf_probe_read_user() helper to
get the value a->b. Without utilizing BTF __user tagging information,
current verifier will assume that a->b is a kernel memory access
and this may generate incorrect result.

Now BTF contains __user information, it can check whether the
pointer points to a user memory or not. If it is, the verifier
can reject the program and force users to use bpf_probe_read_user()
helper explicitly.

In the future, we can easily extend btf_add_space for other
address space tagging, for example, rcu/percpu etc.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154606.654961-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 11 insertions, 3 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e3b82ce51445..6eb0b180d33b 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -332,7 +332,10 @@ enum bpf_type_flag {
 	 */
 	MEM_ALLOC		= BIT(2 + BPF_BASE_TYPE_BITS),
 
-	__BPF_TYPE_LAST_FLAG	= MEM_ALLOC,
+	/* MEM is in user address space. */
+	MEM_USER		= BIT(3 + BPF_BASE_TYPE_BITS),
+
+	__BPF_TYPE_LAST_FLAG	= MEM_USER,
 };
 
 /* Max number of base types. */
@@ -588,7 +591,7 @@ struct bpf_verifier_ops {
 				 const struct btf *btf,
 				 const struct btf_type *t, int off, int size,
 				 enum bpf_access_type atype,
-				 u32 *next_btf_id);
+				 u32 *next_btf_id, enum bpf_type_flag *flag);
 };
 
 struct bpf_prog_offload_ops {
@@ -1780,7 +1783,7 @@ static inline bool bpf_tracing_btf_ctx_access(int off, int size,
 int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
 		      const struct btf_type *t, int off, int size,
 		      enum bpf_access_type atype,
-		      u32 *next_btf_id);
+		      u32 *next_btf_id, enum bpf_type_flag *flag);
 bool btf_struct_ids_match(struct bpf_verifier_log *log,
 			  const struct btf *btf, u32 id, int off,
 			  const struct btf *need_btf, u32 need_type_id);
diff --git a/include/linux/btf.h b/include/linux/btf.h
index b12cfe3b12bb..f6c43dd513fa 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -238,6 +238,11 @@ static inline bool btf_type_is_var(const struct btf_type *t)
 	return BTF_INFO_KIND(t->info) == BTF_KIND_VAR;
 }
 
+static inline bool btf_type_is_type_tag(const struct btf_type *t)
+{
+	return BTF_INFO_KIND(t->info) == BTF_KIND_TYPE_TAG;
+}
+
 /* union is only a special case of struct:
  * all its offsetof(member) == 0
  */