The code pattern of memcpy(dst, src, strlen(src)) is almost always
wrong. In this case it is wrong because it leaves memory uninitialized
if it is less than sizeof(ni->name), and overflows ni->name when longer.

Normally strtomem_pad() could be used here, but since ni->name is a
trailing array in struct hci_mon_new_index, compilers that don't support
-fstrict-flex-arrays=3 can't tell how large this array is via
__builtin_object_size(). Instead, open-code the helper and use sizeof()
since it will work correctly.

Additionally mark ni->name as __nonstring since it appears to not be a
%NUL terminated C string.

Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Edward AD <twuufnxlz@gmail.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-bluetooth@vger.kernel.org
Cc: netdev@vger.kernel.org
Fixes: 78480de55a96 ("Bluetooth: hci_sock: fix slab oob read in create_monitor_event")
Link: https://lore.kernel.org/lkml/202310110908.F2639D3276@keescook/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/net/bluetooth/hci_mon.h | 2 +-
 net/bluetooth/hci_sock.c        | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/net/bluetooth/hci_mon.h b/include/net/bluetooth/hci_mon.h
index 2d5fcda1bcd0..082f89531b88 100644
--- a/include/net/bluetooth/hci_mon.h
+++ b/include/net/bluetooth/hci_mon.h
@@ -56,7 +56,7 @@ struct hci_mon_new_index {
 	__u8		type;
 	__u8		bus;
 	bdaddr_t	bdaddr;
-	char		name[8];
+	char		name[8] __nonstring;
 } __packed;
 #define HCI_MON_NEW_INDEX_SIZE 16
 
diff --git a/net/bluetooth/hci_sock.c b/net/bluetooth/hci_sock.c
index 72abe54c45dd..3e7cd330d731 100644
--- a/net/bluetooth/hci_sock.c
+++ b/net/bluetooth/hci_sock.c
@@ -488,7 +488,8 @@ static struct sk_buff *create_monitor_event(struct hci_dev *hdev, int event)
 		ni->type = hdev->dev_type;
 		ni->bus = hdev->bus;
 		bacpy(&ni->bdaddr, &hdev->bdaddr);
-		memcpy(ni->name, hdev->name, strlen(hdev->name));
+		memcpy_and_pad(ni->name, sizeof(ni->name), hdev->name,
+			       strnlen(hdev->name, sizeof(ni->name)), '\0');
 
 		opcode = cpu_to_le16(HCI_MON_NEW_INDEX);
 		break;
-- 
2.34.1

Hello:

This patch was applied to bluetooth/bluetooth-next.git (master)
by Luiz Augusto von Dentz <luiz.von.dentz@intel.com>:

On Wed, 11 Oct 2023 09:31:44 -0700 you wrote:
> The code pattern of memcpy(dst, src, strlen(src)) is almost always
> wrong. In this case it is wrong because it leaves memory uninitialized
> if it is less than sizeof(ni->name), and overflows ni->name when longer.
> 
> Normally strtomem_pad() could be used here, but since ni->name is a
> trailing array in struct hci_mon_new_index, compilers that don't support
> -fstrict-flex-arrays=3 can't tell how large this array is via
> __builtin_object_size(). Instead, open-code the helper and use sizeof()
> since it will work correctly.
> 
> [...]

Here is the summary with links:
  - Bluetooth: hci_sock: Correctly bounds check and pad HCI_MON_NEW_INDEX name
    https://git.kernel.org/bluetooth/bluetooth-next/c/fbd34cc57479

You are awesome, thank you!
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