Re: [PATCH v2][next] Bluetooth: hci_conn, hci_sync: Use __counted_by() in multiple structs and avoid -Wfamnae warnings

Kees Cook
2024-04-29T18:16:13Z

On Fri, Apr 26, 2024 at 04:52:46PM -0600, Gustavo A. R. Silva wrote:
> 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).
>
> Also, -Wflex-array-member-not-at-end is coming in GCC-14, and we are
> getting ready to enable it globally.
>
> So, use the `DEFINE_FLEX()` helper for multiple on-stack definitions
> of a flexible structure where the size of the flexible-array member
> is known at compile-time, and refactor the rest of the code,
> accordingly.
>
> Notice that, due to the use of `__counted_by()` in `struct
> hci_cp_le_create_cis`, the for loop in function `hci_cs_le_create_cis()`
> had to be modified. Once the index `i`, through which `cp->cis[i]` is
> accessed, falls in the interval [0, cp->num_cis), `cp->num_cis` cannot
> be decremented all the way down to zero while accessing `cp->cis[]`:
>
> net/bluetooth/hci_event.c:4310:
> 4310 for (i = 0; cp->num_cis; cp->num_cis--, i++) {
> ...
> 4314 handle = __le16_to_cpu(cp->cis[i].cis_handle);
>
> otherwise, only half (one iteration before `cp->num_cis == i`) or half
> plus one (one iteration before `cp->num_cis < i`) of the items in the
> array will be accessed before running into an out-of-bounds issue. So,
> in order to avoid this, set `cp->num_cis` to zero just after the for
> loop.
>
> Also, make use of `aux_num_cis` variable to update `cmd->num_cis` after
> a `list_for_each_entry_rcu()` loop.
>
> With these changes, fix the following warnings:
> net/bluetooth/hci_sync.c:1239:56: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> net/bluetooth/hci_sync.c:1415:51: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> net/bluetooth/hci_sync.c:1731:51: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> net/bluetooth/hci_sync.c:6497:45: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>
> Link: https://github.com/KSPP/linux/issues/202
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> Changes in v2:
> - Update `cmd->num_cis` after `list_for_each_entry_rcu()` loop.
>
> v1:
> - Link: https://lore.kernel.org/linux-hardening/ZiwqqZCa7PK9bzCX@neat/
>
> include/net/bluetooth/hci.h | 8 ++--
> net/bluetooth/hci_event.c | 3 +-
> net/bluetooth/hci_sync.c | 84 +++++++++++++++----------------------
> 3 files changed, 40 insertions(+), 55 deletions(-)
>
> diff --git a/include/net/bluetooth/hci.h b/include/net/bluetooth/hci.h
> index fe23e862921d..c4c6b8810701 100644
> --- a/include/net/bluetooth/hci.h
> +++ b/include/net/bluetooth/hci.h
> @@ -2026,7 +2026,7 @@ struct hci_cp_le_set_ext_adv_data {
> __u8 operation;
> __u8 frag_pref;
> __u8 length;
> - __u8 data[];
> + __u8 data[] __counted_by(length);
> } __packed;

I noticed some of the other structs here aren't flexible arrays, so it
made me go take a look at these ones. I see that the only user of struct
hci_cp_le_set_ext_adv_data uses a fixed-size array:

struct {
struct hci_cp_le_set_ext_adv_data cp;
u8 data[HCI_MAX_EXT_AD_LENGTH];
} pdu;

Let's just change this from a flex array to a fixed-size array?

>
> #define HCI_OP_LE_SET_EXT_SCAN_RSP_DATA 0x2038
> @@ -2035,7 +2035,7 @@ struct hci_cp_le_set_ext_scan_rsp_data {
> __u8 operation;
> __u8 frag_pref;
> __u8 length;
> - __u8 data[];
> + __u8 data[] __counted_by(length);
> } __packed;

Same for this one:

struct {
struct hci_cp_le_set_ext_scan_rsp_data cp;
u8 data[HCI_MAX_EXT_AD_LENGTH];
} pdu;

>
> #define HCI_OP_LE_SET_EXT_ADV_ENABLE 0x2039
> @@ -2061,7 +2061,7 @@ struct hci_cp_le_set_per_adv_data {
> __u8 handle;
> __u8 operation;
> __u8 length;
> - __u8 data[];
> + __u8 data[] __counted_by(length);
> } __packed;

And this one. :P

struct {
struct hci_cp_le_set_per_adv_data cp;
u8 data[HCI_MAX_PER_AD_LENGTH];
} pdu;

>
> #define HCI_OP_LE_SET_PER_ADV_ENABLE 0x2040
> @@ -2162,7 +2162,7 @@ struct hci_cis {
>
> struct hci_cp_le_create_cis {
> __u8 num_cis;
> - struct hci_cis cis[];
> + struct hci_cis cis[] __counted_by(num_cis);
> } __packed;

This one isn't as obvious, so I'd say keep your changes for this one.

>
> #define HCI_OP_LE_REMOVE_CIG 0x2065
> diff --git a/net/bluetooth/hci_event.c b/net/bluetooth/hci_event.c
> index 9a38e155537e..9a7ca084302e 100644
> --- a/net/bluetooth/hci_event.c
> +++ b/net/bluetooth/hci_event.c
> @@ -4307,7 +4307,7 @@ static void hci_cs_le_create_cis(struct hci_dev *hdev, u8 status)
> hci_dev_lock(hdev);
>
> /* Remove connection if command failed */
> - for (i = 0; cp->num_cis; cp->num_cis--, i++) {
> + for (i = 0; i < cp->num_cis; i++) {
> struct hci_conn *conn;
> u16 handle;
>
> @@ -4323,6 +4323,7 @@ static void hci_cs_le_create_cis(struct hci_dev *hdev, u8 status)
> hci_conn_del(conn);
> }
> }
> + cp->num_cis = 0;

Yeah, this loop never leaves early, and if it did, it processing the
array forward, so having num_cis reduced during the loop iteration
doesn't make sense. What you have looks right to me!

>
> if (pending)
> hci_le_create_cis_pending(hdev);
> diff --git a/net/bluetooth/hci_sync.c b/net/bluetooth/hci_sync.c
> index 9092b4d59545..6e15594d3565 100644
> --- a/net/bluetooth/hci_sync.c
> +++ b/net/bluetooth/hci_sync.c
> @@ -1235,31 +1235,27 @@ int hci_setup_ext_adv_instance_sync(struct hci_dev *hdev, u8 instance)
>
> static int hci_set_ext_scan_rsp_data_sync(struct hci_dev *hdev, u8 instance)
> {
> - struct {
> - struct hci_cp_le_set_ext_scan_rsp_data cp;
> - u8 data[HCI_MAX_EXT_AD_LENGTH];
> - } pdu;
> + DEFINE_FLEX(struct hci_cp_le_set_ext_scan_rsp_data, pdu, data, length,
> + HCI_MAX_EXT_AD_LENGTH);
> u8 len;
> struct adv_info *adv = NULL;
> int err;
>
> - memset(&pdu, 0, sizeof(pdu));

These become much easier, just:

struct hci_cp_le_set_ext_scan_rsp_data cp = { };

etc...

--
Kees Cook