Re: [PATCH-for-6.2] hw/nvme/ctrl: Fix buffer overrun (CVE-2021-3947)

[Philippe Mathieu-Daudé]

On 11/11/21 19:08, Klaus Jensen wrote:
> On Nov 11 16:31, Philippe Mathieu-Daudé wrote:
>> Both 'buf_len' and 'off' arguments are under guest control.
>> Since nvme_c2h() doesn't check out of boundary access, the
>> caller must check for eventual buffer overrun on 'trans_len'.
>>
>> Cc: qemu-stable@nongnu.org
>> Reported-by: Qiuhao Li <Qiuhao.Li@outlook.com>
>> Fixes: f432fdfa121 ("support changed namespace asynchronous event")
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  hw/nvme/ctrl.c | 22 ++++++++++++----------
>>  1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c
>> index 6a571d18cfa..634b290e069 100644
>> --- a/hw/nvme/ctrl.c
>> +++ b/hw/nvme/ctrl.c
>> @@ -4072,7 +4072,8 @@ static uint16_t nvme_smart_info(NvmeCtrl *n, uint8_t 
>> rae, uint32_t buf_len,
>>      NvmeNamespace *ns;
>>      time_t current_ms;
>>  
>> -    if (off >= sizeof(smart)) {
>> +    trans_len = MIN(sizeof(smart) - off, buf_len);
>> +    if (trans_len >= sizeof(smart)) {
>>          return NVME_INVALID_FIELD | NVME_DNR;
>>      }
>>  
>> @@ -4094,7 +4095,6 @@ static uint16_t nvme_smart_info(NvmeCtrl *n, uint8_t 
>> rae, uint32_t buf_len,
>>          }
>>      }
>>  
>> -    trans_len = MIN(sizeof(smart) - off, buf_len);
>>      smart.critical_warning = n->smart_critical_warning;
>>  
>>      smart.data_units_read[0] = cpu_to_le64(DIV_ROUND_UP(stats.units_read,
> 
> Uhm. Hehe.
> 
> This "fix" breaks all log pages. Take smart_info as an example. Say the
> offset is zero and the buffer length is 512. The transfer length
> (trans_len) then becomes 512 and it ends up returning Invalid Field in
> Command because trans_len equals sizeof(smart).
> 
> Worse, this "fix" actually *introduce* oob's all over the place if
> 
>     off > sizeof(smart) && buf_len < sizeof(smart)
> 
> 
> Example
> 
>    sizeof(smart) = 512
>    off = 516 (must be dword aligned to get to this spot)
>    buf_len = 4 (same, but is always aligned)
>    => trans_len = min(512 - 516, 4) = 4
>    if (1 >= 512) => false
> 
> And we end up with
> 
>    nvme_c2h(n, &smart + 516, 4, req);

Doh, I missed the '+ off' part in the nvme_c2h() call...