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

[Laszlo Ersek]

On 01/09/18 14:33, 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.

Thanks
Laszlo