Re: [PATCH] ossaudio: fix out of bounds write

[Volker Rümelin]

> On 7/7/20 8:08 PM, Volker Rümelin wrote:
>> In function oss_read() a read error currently does not exit the
>> read loop. With no data to read the variable pos will quickly
>> underflow and a subsequent successful read overwrites memory
>> outside the buffer. This patch adds the missing break statement
>> to the error path of the function.
> Correct, but ...
>
>> To reproduce start qemu with -audiodev oss,id=audio0 and in the
>> guest start audio recording. After some time this will trigger
>> an exception.
>>
>> Fixes: 3ba4066d08 "ossaudio: port to the new audio backend api"
>>
>> Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
>> ---
>>  audio/ossaudio.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/audio/ossaudio.c b/audio/ossaudio.c
>> index f88d076ec2..a7dcaa31ad 100644
>> --- a/audio/ossaudio.c
>> +++ b/audio/ossaudio.c
>> @@ -691,6 +691,7 @@ static size_t oss_read(HWVoiceIn *hw, void *buf, size_t 
>> len)
>>                             len, dst);
>>                  break;
>>              }
>> +            break;
>>          }
>>  
>>          pos += nread;
> ... now pos += -1, then the size returned misses the last byte.
>
Hi Philippe,

no, the added break breaks the while loop. The next executed instruction after 
this break is the return pos statement not pos += nread.

With best regards,
Volker