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 btf_id at verification
time.
This patch adds verifier 'poison' functionality to BPF_PTR_POISON in
order to prepare for expanded use of the value to poison ret- and
arg-btf_id in ongoing work, namely rbtree and linked list patchsets
[0, 1]. 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 arg 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 include/linux/posion.h header and
remove unnecessary shift.
[0]: lore.kernel.org/bpf/20220830172759.4069786-1-davemarchevsky@xxxxxx
[1]: lore.kernel.org/bpf/20220904204145.3089-1-memxor@xxxxxxxxx
Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
---
This patch was originally part of [0] and is used in [1]. Pulling out
and submitting separately as discussed in patch 18's thread in [1].
To validate, comment out the 'if' test and block preceding the 'else's
changed by this patch. (Either or both will work). The poisoned ret and
arg types for bpf_kptr_xchg are overwritten in these 'if's, so
commenting out the overwriting will result in the added behavior causing
progs which use bpf_kptr_xchg to fail to pass verifier. Can confirm by
doing './test_progs -t map_kptr' .
include/linux/poison.h | 3 +++
kernel/bpf/helpers.c | 6 +++---
kernel/bpf/verifier.c | 30 +++++++++++++++++++++++-------
3 files changed, 29 insertions(+), 10 deletions(-)
diff --git a/include/linux/poison.h b/include/linux/poison.h
index d62ef5a6b4e9..2d3249eb0e62 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 *)(0xeB9FUL + POISON_POINTER_DELTA))
+
#endif
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index fc08035f14ed..41aeaf3862ec 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>
@@ -1376,10 +1377,9 @@ 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 c259d734f863..8c6fbcd0afaf 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"
@@ -5782,13 +5783,22 @@ 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,
- 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;
+ } else {
+ if (arg_btf_id == BPF_PTR_POISON) {
+ verbose(env, "verifier internal error:");
+ verbose(env, "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;
+ }
}
}
@@ -7457,6 +7467,12 @@ 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 {
+ if (fn->ret_btf_id == BPF_PTR_POISON) {
+ verbose(env, "verifier internal error:");
+ verbose(env, "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;
}
--
2.30.2
