[RFCv2 PATCH bpf-next 02/18] bpf: Add verifier check for BPF_PTR_POISON retval and arg

[Dave Marchevsky <davemarchevsky@fb.com>]

BPF_PTR_POISON was added in commit c0a5a21c25f37 ("bpf: Allow storing
referenced kptr in map") to denote a bpf_func_proto btf_id which the
verifier will replace with a dynamically-determined one at verification
time.

This patch adds 'poison' functionality to BPF_PTR_POISON in order to
prepare for expanded use of the value to poison ret- and arg-btf_id in
further patches of this series. Specifically, when the verifier checks
helper calls, it assumes that BPF_PTR_POISON'ed ret type will be
replaced with a valid type before - or in lieu of - the default
ret_btf_id logic. Similarly for argument btf_id.

If poisoned btf_id reaches default handling block for either, consider
this a verifier internal error and fail verification. Otherwise a helper
w/ poisoned btf_id but no verifier logic replacing the type will cause a
crash as the invalid pointer is dereferenced.

Also move BPF_PTR_POISON to existing <linux/posion.h> header and replace
the hardcoded value with something that looks slightly like BPF (0xB4F).

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
---
 include/linux/poison.h |  3 +++
 kernel/bpf/helpers.c   |  6 +++---
 kernel/bpf/verifier.c  | 28 ++++++++++++++++++++++------
 3 files changed, 28 insertions(+), 9 deletions(-)

diff --git a/include/linux/poison.h b/include/linux/poison.h
index d62ef5a6b4e9..599995d29fe4 100644
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -81,4 +81,7 @@
 /********** net/core/page_pool.c **********/
 #define PP_SIGNATURE		(0x40 + POISON_POINTER_DELTA)
 
+/********** kernel/bpf/ **********/
+#define BPF_PTR_POISON ((void *)(0xB4FUL + POISON_POINTER_DELTA))
+
 #endif
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 3c1b9bbcf971..6de4dedf45c0 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -15,6 +15,7 @@
 #include <linux/ctype.h>
 #include <linux/jiffies.h>
 #include <linux/pid_namespace.h>
+#include <linux/poison.h>
 #include <linux/proc_ns.h>
 #include <linux/security.h>
 #include <linux/btf_ids.h>
@@ -1410,10 +1411,9 @@ BPF_CALL_2(bpf_kptr_xchg, void *, map_value, void *, ptr)
 }
 
 /* Unlike other PTR_TO_BTF_ID helpers the btf_id in bpf_kptr_xchg()
- * helper is determined dynamically by the verifier.
+ * helper is determined dynamically by the verifier. Use BPF_PTR_POISON to
+ * denote type that verifier will determine.
  */
-#define BPF_PTR_POISON ((void *)((0xeB9FUL << 2) + POISON_POINTER_DELTA))
-
 static const struct bpf_func_proto bpf_kptr_xchg_proto = {
 	.func         = bpf_kptr_xchg,
 	.gpl_only     = false,
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 68bfa7c28048..ee5b57140c73 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -23,6 +23,7 @@
 #include <linux/error-injection.h>
 #include <linux/bpf_lsm.h>
 #include <linux/btf_ids.h>
+#include <linux/poison.h>
 
 #include "disasm.h"
 
@@ -5764,13 +5765,21 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 		if (meta->func_id == BPF_FUNC_kptr_xchg) {
 			if (map_kptr_match_type(env, meta->kptr_off_desc, reg, regno))
 				return -EACCES;
-		} else if (!btf_struct_ids_match(&env->log, reg->btf, reg->btf_id, reg->off,
+		} else {
+			if (arg_btf_id == BPF_PTR_POISON) {
+				verbose(env, "verifier internal error: R%d has "
+					"non-overwritten BPF_PTR_POISON type\n", regno);
+				return -EACCES;
+			}
+
+			if (!btf_struct_ids_match(&env->log, reg->btf, reg->btf_id, reg->off,
 						 btf_vmlinux, *arg_btf_id,
 						 strict_type_match)) {
-			verbose(env, "R%d is of type %s but %s is expected\n",
-				regno, kernel_type_name(reg->btf, reg->btf_id),
-				kernel_type_name(btf_vmlinux, *arg_btf_id));
-			return -EACCES;
+				verbose(env, "R%d is of type %s but %s is expected\n",
+					regno, kernel_type_name(reg->btf, reg->btf_id),
+					kernel_type_name(btf_vmlinux, *arg_btf_id));
+				return -EACCES;
+			}
 		}
 	}
 
@@ -7454,7 +7463,14 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 			ret_btf = meta.kptr_off_desc->kptr.btf;
 			ret_btf_id = meta.kptr_off_desc->kptr.btf_id;
 		} else {
-			ret_btf = btf_vmlinux;
+			if (fn->ret_btf_id == BPF_PTR_POISON) {
+				verbose(env, "verifier internal error: func %s "
+					"has non-overwritten "
+					"BPF_PTR_POISON return type\n",
+					func_id_name(func_id));
+				return -EINVAL;
+			}
+		        ret_btf = btf_vmlinux;
 			ret_btf_id = *fn->ret_btf_id;
 		}
 		if (ret_btf_id == 0) {
-- 
2.30.2