[PATCH] netlink: Annotate struct netlink_policy_dump_state with __counted_by

From: Kees Cook
Date: Tue Oct 03 2023 - 19:21:13 EST


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" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Johannes Berg <johannes.berg@xxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
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