Re: [PATCH-for-5.0 v2 1/4] Revert "prevent crash when executing guest-file-read with large count"

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 3/31/20 4:12 PM, Daniel P. Berrangé wrote:
> On Tue, Mar 31, 2020 at 04:06:35PM +0200, Philippe Mathieu-Daudé wrote:
>> By using g_try_malloc() instead of g_malloc() the qemu-guest-agent
>> Denial-of-Service attack referred in commit 807e2b6fce is reduced,
>> but still triggerable:
> 
> As explained previously, I believe there is *no* denial of service
> attack here. The described scenario is just a user hurting themselves
> by intentionally telling QEMU not to limit the amount of data returned.

Yes. Do you mind updating the BZ, eventually marking as NOTABUG? Then I 
can adapt the patch descriptions.
https://bugzilla.redhat.com/show_bug.cgi?id=1594054

> 
>>
>>    - bisect file size S until g_try_malloc(S) fails,
>>    - use S - 1:
>>      g_try_malloc(S - 1) succeeds, but g_new0() few lines later will
>>      fail.
>>
>>   346     buf = g_try_malloc0(count + 1);
>>   347     if (!buf) {
>>   348         error_setg(errp,
>>   349                    "failed to allocate sufficient memory "
>>   350                    "to complete the requested service");
>>   351         return NULL;
>>   352     }
>>   353     is_ok = ReadFile(fh, buf, count, &read_count, NULL);
>>   354     if (!is_ok) {
>>   355         error_setg_win32(errp, GetLastError(), "failed to read file");
>>   356         slog("guest-file-read failed, handle %" PRId64, handle);
>>   357     } else {
>>   358         buf[read_count] = 0;
>>   359         read_data = g_new0(GuestFileRead, 1);
>>                           ^^^^^^
>>
>> Instead we are going to put a low hard limit on 'count' in the next
>> commits.
>> This reverts commit 807e2b6fce022707418bc8f61c069d91c613b3d2.
>>
>> Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>   qga/commands-win32.c | 8 +-------
>>   1 file changed, 1 insertion(+), 7 deletions(-)
>>
>> diff --git a/qga/commands-win32.c b/qga/commands-win32.c
>> index b49920e201..46cea7d1d9 100644
>> --- a/qga/commands-win32.c
>> +++ b/qga/commands-win32.c
>> @@ -343,13 +343,7 @@ GuestFileRead *qmp_guest_file_read(int64_t handle, bool has_count,
>>       }
>>   
>>       fh = gfh->fh;
>> -    buf = g_try_malloc0(count + 1);
>> -    if (!buf) {
>> -        error_setg(errp,
>> -                   "failed to allocate sufficient memory "
>> -                   "to complete the requested service");
>> -        return NULL;
>> -    }
>> +    buf = g_malloc0(count + 1);
>>       is_ok = ReadFile(fh, buf, count, &read_count, NULL);
>>       if (!is_ok) {
>>           error_setg_win32(errp, GetLastError(), "failed to read file");
>> -- 
>> 2.21.1
>>
> 
> Regards,
> Daniel
>