Re: [Qemu-stable] [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg slots are used

[Laszlo Ersek]

On 01/09/18 14:35, Laszlo Ersek wrote:
> On 01/09/18 14:33, Laszlo Ersek wrote:
>> On 01/09/18 14:18, Marcel Apfelbaum wrote:
>>> On 09/01/2018 15:09, Laszlo Ersek wrote:
>>>
>>> Hi Laszlo,
>>>
>>> I'll respond first to this mail' I'll take my time with the rest :)
>>>
>>>> On 01/08/18 22:50, Marcel Apfelbaum wrote:
>>>>> When all the fw_cfg slots are used, a write is made outside the
>>>>> bounds of the fw_cfg files array as part of the sort algorithm.
>>>>>
>>>>> Fix it by avoiding an unnecessary array element move.
>>>>> Fix also an assert while at it.
>>>>>
>>>>> Signed-off-by: Marcel Apfelbaum <address@hidden>
>>>>> ---
>>>>>   hw/nvram/fw_cfg.c | 6 ++++--
>>>>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
>>>>> index 753ac0e4ea..4313484b21 100644
>>>>> --- a/hw/nvram/fw_cfg.c
>>>>> +++ b/hw/nvram/fw_cfg.c
>>>>> @@ -784,7 +784,7 @@ void fw_cfg_add_file_callback(FWCfgState *s, 
>>>>> const char *filename,
>>>>>        * index and "i - 1" is the one being copied from, thus the
>>>>>        * unusual start and end in the for statement.
>>>>>        */
>>>>> -    for (i = count + 1; i > index; i--) {
>>>>> +    for (i = count; i > index; i--) {
>>>>>           s->files->f[i] = s->files->f[i - 1];
>>>>>           s->files->f[i].select = cpu_to_be16(FW_CFG_FILE_FIRST + i);
>>>>>           s->entries[0][FW_CFG_FILE_FIRST + i] =
>>>>
>>>> This hunk looks correct to me.
>>>
>>> After my change or before?
>>
>> Well, the source code doesn't have "hunks", patches have hunks. :)
>>
>> So, I meant, this part of your patch was correct, IMO.
>>
>>>
>>> I think I am right.
>>> At this point we have "count" elements in the array.
>>> That means the last element in the array is at arr[count - 1].
>>> We want to make room for the new element at index, so we move
>>> all the elements from index to index + 1.
>>>
>>> The first element we should move is arr[count - 1] to arr[count].
>>> But the code moved arr[count] to arr [count + 1].
>>> This move is not needed.
>>>
>>>
>>>  We currently have count elements in the
>>>> array, so we cannot normally access the element *at* count. However, we
>>>> are extending the array right now, therefore we can assign (store) the
>>>> element at count (and then we'll increment count later). But accessing
>>>> an element at (count+1) is wrong.
>>>>
>>>>> @@ -833,7 +833,6 @@ void *fw_cfg_modify_file(FWCfgState *s, const
>>>>> char *filename,
>>>>>       assert(s->files);
>>>>>         index = be32_to_cpu(s->files->count);
>>>>> -    assert(index < fw_cfg_file_slots(s));
>>>>>         for (i = 0; i < index; i++) {
>>>>>           if (strcmp(filename, s->files->f[i].name) == 0) {
>>>>> @@ -843,6 +842,9 @@ void *fw_cfg_modify_file(FWCfgState *s, const
>>>>> char *filename,
>>>>>               return ptr;
>>>>>           }
>>>>>       }
>>>>> +
>>>>> +    assert(index < fw_cfg_file_slots(s));
>>>>> +
>>>>>       /* add new one */
>>>>>       fw_cfg_add_file_callback(s, filename, NULL, NULL, NULL, data,
>>>>> len, true);
>>>>>       return NULL;
>>>>>
>>>>
>>>> I think I agree with Marc-André here, when I say, replace the assert
>>>> with a comment instead? (About the fact that fw_cfg_add_file_callback()
>>>> will assert(), *if* we reach that far.)
>>>
>>> Hmm, what should we add to the comment? "We lost, brace for impact :)"
>>>
>>> My point, if we are going to abort, let's abort as early as we can.
>>> But if is a consensus, I'll get rid of it.
>>
>> No, it's going to be another assert, just later. Assume that at this
>> point we have (index == fw_cfg_file_slots(s)), because the function
>> didn't find the element to modify, so it decides to add a new one, but
>> also we do not have room for the new one. So, with the suggested removal
>> of the assert, we call fw_cfg_add_file_callback().
>>
>> Then, fw_cfg_add_file_callback() does:
>>
>>     if (!s->files) {
>>         dsize = sizeof(uint32_t) + sizeof(FWCfgFile) * fw_cfg_file_slots(s);
>>         s->files = g_malloc0(dsize);
>>         fw_cfg_add_bytes(s, FW_CFG_FILE_DIR, s->files, dsize);
>>     }
>>
>>     count = be32_to_cpu(s->files->count);
>>     assert(count < fw_cfg_file_slots(s));
>>
>> The (!s->files) condition is expected to eval to false (our table is
>> full, so we do have a table).
>>
>> And then, the assert() below the "if" will fire.
>>
>> Am I missing something?
> 
> Hm, OK, your point was, abort as *early* as we can.
> 
> I guess you are not wrong :) I'm fine either way, then.

... which means:

Reviewed-by: Laszlo Ersek <address@hidden>

(sorry, need more sleep)