Bluetooth: btintel: Check dsbr size from EFI variable

Since the size of struct btintel_dsbr is already known, we can just
start there instead of querying the EFI variable size. If the final
result doesn't match what we expect also fail. This fixes a stack buffer
overflow when the EFI variable is larger than struct btintel_dsbr.

Reported-by: zepta <z3ptaa@gmail.com>
Closes: https://lore.kernel.org/all/CAPBS6KoaWV9=dtjTESZiU6KK__OZX0KpDk-=JEH8jCHFLUYv3Q@mail.gmail.com
Fixes: eb9e749 ("Bluetooth: btintel: Allow configuring drive strength of BRI")
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

Author: kees

drivers/bluetooth/btintel.c

@@ -2719,7 +2719,7 @@ static int btintel_uefi_get_dsbr(u32 *dsbr_var)
 	} __packed data;
 
 	efi_status_t status;
-	unsigned long data_size = 0;
+	unsigned long data_size = sizeof(data);
 	efi_guid_t guid = EFI_GUID(0xe65d8884, 0xd4af, 0x4b20, 0x8d, 0x03,
 				   0x77, 0x2e, 0xcc, 0x3d, 0xa5, 0x31);
 
@@ -2729,16 +2729,10 @@ static int btintel_uefi_get_dsbr(u32 *dsbr_var)
 	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE))
 		return -EOPNOTSUPP;
 
-	status = efi.get_variable(BTINTEL_EFI_DSBR, &guid, NULL, &data_size,
-				  NULL);
-
-	if (status != EFI_BUFFER_TOO_SMALL || !data_size)
-		return -EIO;
-
 	status = efi.get_variable(BTINTEL_EFI_DSBR, &guid, NULL, &data_size,
 				  &data);
 
-	if (status != EFI_SUCCESS)
+	if (status != EFI_SUCCESS || data_size != sizeof(data))
 		return -ENXIO;
 
 	*dsbr_var = data.dsbr;