Subject: [PATCH] netlink: Annotate struct netlink_policy_dump_state with __counted_by

From: Kees Cook <[email-masked]>
Date: Tue, 3 Oct 2023 16:21:02 -0700

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct netlink_policy_dump_state.

Additionally update the size of the usage array length before accessing
it. This requires remembering the old size for the memset() and later
assignments.

Cc: "David S. Miller" <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Cc: Jakub Kicinski <[email-masked]>
Cc: Paolo Abeni <[email-masked]>
Cc: Johannes Berg <[email-masked]>
Cc: [email-masked]
Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]
Signed-off-by: Kees Cook <[email-masked]>
---
 net/netlink/policy.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/net/netlink/policy.c b/net/netlink/policy.c
index 87e3de0fde89..e2f111edf66c 100644
--- a/net/netlink/policy.c
+++ b/net/netlink/policy.c
@@ -21,7 +21,7 @@ struct netlink_policy_dump_state {
 	struct {
 		const struct nla_policy *policy;
 		unsigned int maxtype;
-	} policies[];
+	} policies[] __counted_by(n_alloc);
 };
 
 static int add_policy(struct netlink_policy_dump_state **statep,
@@ -29,7 +29,7 @@ static int add_policy(struct netlink_policy_dump_state **statep,
 		      unsigned int maxtype)
 {
 	struct netlink_policy_dump_state *state = *statep;
-	unsigned int n_alloc, i;
+	unsigned int old_n_alloc, n_alloc, i;
 
 	if (!policy || !maxtype)
 		return 0;
@@ -52,12 +52,13 @@ static int add_policy(struct netlink_policy_dump_state **statep,
 	if (!state)
 		return -ENOMEM;
 
-	memset(&state->policies[state->n_alloc], 0,
-	       flex_array_size(state, policies, n_alloc - state->n_alloc));
-
-	state->policies[state->n_alloc].policy = policy;
-	state->policies[state->n_alloc].maxtype = maxtype;
+	old_n_alloc = state->n_alloc;
 	state->n_alloc = n_alloc;
+	memset(&state->policies[old_n_alloc], 0,
+	       flex_array_size(state, policies, n_alloc - old_n_alloc));
+
+	state->policies[old_n_alloc].policy = policy;
+	state->policies[old_n_alloc].maxtype = maxtype;
 	*statep = state;
 
 	return 0;
-- 
2.34.1