linux-kernel - Re: [PATCH 2/4] ACPI: battery: Fix buffer overread if not NUL-terminated

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Message-ID: <90f1270d-3c37-088b-98ac-a08caba685e8@gmx.de>
Date:   Tue, 17 Jan 2023 22:01:17 +0100
From:   Armin Wolf <W_Armin@....de>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     lenb@...nel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] ACPI: battery: Fix buffer overread if not
 NUL-terminated

Am 17.01.23 um 15:42 schrieb Rafael J. Wysocki:

> On Sat, Jan 14, 2023 at 9:51 AM Armin Wolf <W_Armin@....de> wrote:
>> If the buffer containing string data is not NUL-terminated
>> (which is perfectly legal according to the ACPI specification),
>> the acpi battery driver might not honor its length.
> Note that this is about extracting package entries of type ACPI_TYPE_BUFFER.
>
> And please spell ACPI in capitals.
>
>> Fix this by limiting the amount of data to be copied to
>> the buffer length while also using strscpy() to make sure
>> that the resulting string is always NUL-terminated.
> OK
>
>> Also use '\0' instead of a plain 0.
> Why?  It's a u8, not a char.
>
>> Signed-off-by: Armin Wolf <W_Armin@....de>
>> ---
>>   drivers/acpi/battery.c | 23 ++++++++++++++++-------
>>   1 file changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/acpi/battery.c b/drivers/acpi/battery.c
>> index fb64bd217d82..9f6daa9f2010 100644
>> --- a/drivers/acpi/battery.c
>> +++ b/drivers/acpi/battery.c
>> @@ -438,15 +438,24 @@ static int extract_package(struct acpi_battery *battery,
>>                  if (offsets[i].mode) {
>>                          u8 *ptr = (u8 *)battery + offsets[i].offset;
> I would add
>
> u32 len = 32;
>
>> -                       if (element->type == ACPI_TYPE_STRING ||
>> -                           element->type == ACPI_TYPE_BUFFER)
>> +                       switch (element->type) {
> And here I would do
>
> case ACPI_TYPE_BUFFER:
>          if (len > element->buffer.length + 1)
>                  len = element->buffer.length + 1;
>
>          fallthrough;
> case ACPI_TYPE_STRING:
>          strscpy(ptr, element->buffer.pointer, len);
>          break;
> case ACPI_TYPE_INTEGER:
>
> and so on.

But wouldn't this cause the ACPI string object to be accessed the wrong way
(buffer.pointer instead of string.pointer)?

Armin Wolf

>> +                       case ACPI_TYPE_STRING:
>>                                  strscpy(ptr, element->string.pointer, 32);
>> -                       else if (element->type == ACPI_TYPE_INTEGER) {
>> -                               strncpy(ptr, (u8 *)&element->integer.value,
>> -                                       sizeof(u64));
>> +
>> +                               break;
>> +                       case ACPI_TYPE_BUFFER:
>> +                               strscpy(ptr, element->buffer.pointer,
>> +                                       min_t(u32, element->buffer.length + 1, 32));
>> +
>> +                               break;
>> +                       case ACPI_TYPE_INTEGER:
>> +                               strncpy(ptr, (u8 *)&element->integer.value, sizeof(u64));
>>                                  ptr[sizeof(u64)] = 0;
>> -                       } else
>> -                               *ptr = 0; /* don't have value */
>> +
>> +                               break;
>> +                       default:
>> +                               *ptr = '\0'; /* don't have value */
>> +                       }
>>                  } else {
>>                          int *x = (int *)((u8 *)battery + offsets[i].offset);
>>                          *x = (element->type == ACPI_TYPE_INTEGER) ?
>> --