Re: [PATCH v2 3/8] s390/sclp: rework sclp boundary and length checks

[Collin Walling <walling@linux.ibm.com>]

On 5/18/20 4:50 AM, Janosch Frank wrote:
> On 5/16/20 12:20 AM, Collin Walling wrote:
>> Rework the SCLP boundary check to account for different SCLP commands
>> (eventually) allowing different boundary sizes.
>>
>> Move the length check code into a separate function, and introduce a
>> new function to determine the length of the read SCP data (i.e. the size
>> from the start of the struct to where the CPU entries should begin).
>>
>> Signed-off-by: Collin Walling <walling@linux.ibm.com>
>> ---
>>  hw/s390x/sclp.c | 57 ++++++++++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 49 insertions(+), 8 deletions(-)
>>
>> diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
>> index 2bd618515e..987699e3c4 100644
>> --- a/hw/s390x/sclp.c
>> +++ b/hw/s390x/sclp.c
>> @@ -49,6 +49,34 @@ static inline bool sclp_command_code_valid(uint32_t code)
>>      return false;
>>  }
>>  
>> +static bool sccb_has_valid_boundary(uint64_t sccb_addr, uint32_t code,
>> +                                    SCCBHeader *header)
>> +{
>> +    uint64_t current_len = sccb_addr + be16_to_cpu(header->length);
>> +    uint64_t allowed_len = (sccb_addr & PAGE_MASK) + PAGE_SIZE;
> 
> Those are addresses not length indications and the names should reflect
> that.

True

> Also don't we need to use PAGE_SIZE - 1?
> 

Technically we need to -1 on both sides since length denotes the size of
the sccb in bytes, not the max address.

How about this:

s/current_len/sccb_max_addr
s/allowed_len/sccb_boundary

-1 to sccb_max_addr

Change the check to: sccb_max_addr < sccb_boundary

?

> I'm still trying to wake up, so take this with a grain of salt.
> 

No worries. I appreciate the review nonetheless :)

>> +
>> +    switch (code & SCLP_CMD_CODE_MASK) {
>> +    default:
>> +        if (current_len <= allowed_len) {
>> +            return true;
>> +        }
>> +    }
>> +    header->response_code = cpu_to_be16(SCLP_RC_SCCB_BOUNDARY_VIOLATION);
>> +    return false;
>> +}
>> +
>> +/* Calculates sufficient SCCB length to store a full Read SCP/CPU response */
>> +static bool sccb_has_sufficient_len(SCCB *sccb, int num_cpus, int data_len)
>> +{
>> +    int required_len = data_len + num_cpus * sizeof(CPUEntry);
>> +
>> +    if (be16_to_cpu(sccb->h.length) < required_len) {
>> +        sccb->h.response_code = cpu_to_be16(SCLP_RC_INSUFFICIENT_SCCB_LENGTH);
>> +        return false;
>> +    }
>> +    return true;
>> +}
> 
> Hm, from the function name alone I'd not have expected it to also set
> the response code.
> 

It also sets the required length in the header for an extended-length
sccb. Perhaps this function name doesn't hold up well.

Does sccb_check_sufficient_len make more sense?

I think the same could be said of the boundary check function, which
also sets the response code.

What about setting the response code outside the function, similar to
what sclp_comand_code_valid does?

>> +
>>  static void prepare_cpu_entries(MachineState *ms, CPUEntry *entry, int *count)
>>  {
>>      uint8_t features[SCCB_CPU_FEATURE_LEN] = { 0 };
>> @@ -66,6 +94,16 @@ static void prepare_cpu_entries(MachineState *ms, CPUEntry *entry, int *count)
>>      }
>>  }
>>  
>> +/*
>> + * The data length denotes the start of the struct to where the first
>> + * CPU entry is to be allocated. This value also denotes the offset_cpu
>> + * field.
>> + */
>> +static int get_read_scp_info_data_len(void)
>> +{
>> +    return offsetof(ReadInfo, entries);
>> +}
> 
> Not sure what the policy for this is, but maybe this can go into a
> header file?
> David and Conny will surely make that clear to me :)
> 

Not sure either. If anything it might be a good candidate for an inline
function.

>> +
>>  /* Provide information about the configuration, CPUs and storage */
>>  static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>>  {
>> @@ -74,16 +112,16 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>>      int cpu_count;
>>      int rnsize, rnmax;
>>      IplParameterBlock *ipib = s390_ipl_get_iplb();
>> +    int data_len = get_read_scp_info_data_len();
>>  
>> -    if (be16_to_cpu(sccb->h.length) < (sizeof(ReadInfo) + cpu_count * sizeof(CPUEntry))) {
>> -        sccb->h.response_code = cpu_to_be16(SCLP_RC_INSUFFICIENT_SCCB_LENGTH);
>> +    if (!sccb_has_sufficient_len(sccb, machine->possible_cpus->len, data_len)) {
>>          return;
>>      }
>>  
>>      /* CPU information */
>>      prepare_cpu_entries(machine, read_info->entries, &cpu_count);
>>      read_info->entries_cpu = cpu_to_be16(cpu_count);
>> -    read_info->offset_cpu = cpu_to_be16(offsetof(ReadInfo, entries));
>> +    read_info->offset_cpu = cpu_to_be16(data_len);
>>      read_info->highest_cpu = cpu_to_be16(machine->smp.max_cpus - 1);
>>  
>>      read_info->ibc_val = cpu_to_be32(s390_get_ibc_val());
>> @@ -132,16 +170,16 @@ static void sclp_read_cpu_info(SCLPDevice *sclp, SCCB *sccb)
>>  {
>>      MachineState *machine = MACHINE(qdev_get_machine());
>>      ReadCpuInfo *cpu_info = (ReadCpuInfo *) sccb;
>> +    int data_len = offsetof(ReadCpuInfo, entries);
>>      int cpu_count;
>>  
>> -    if (be16_to_cpu(sccb->h.length) < (sizeof(ReadCpuInfo) + cpu_count * sizeof(CPUEntry))) {
>> -        sccb->h.response_code = cpu_to_be16(SCLP_RC_INSUFFICIENT_SCCB_LENGTH);
>> +    if (!sccb_has_sufficient_len(sccb, machine->possible_cpus->len, data_len)) {
>>          return;
>>      }
>>  
>>      prepare_cpu_entries(machine, cpu_info->entries, &cpu_count);
>>      cpu_info->nr_configured = cpu_to_be16(cpu_count);
>> -    cpu_info->offset_configured = cpu_to_be16(offsetof(ReadCpuInfo, entries));
>> +    cpu_info->offset_configured = cpu_to_be16(data_len);
>>      cpu_info->nr_standby = cpu_to_be16(0);
>>  
>>      /* The standby offset is 16-byte for each CPU */
>> @@ -227,6 +265,10 @@ int sclp_service_call_protected(CPUS390XState *env, uint64_t sccb,
>>          goto out_write;
>>      }
>>  
>> +    if (!sccb_has_valid_boundary(sccb, code, &work_sccb.h)) {
>> +        goto out_write;
>> +    }
>> +
>>      sclp_c->execute(sclp, &work_sccb, code);
>>  out_write:
>>      s390_cpu_pv_mem_write(env_archcpu(env), 0, &work_sccb,
>> @@ -272,8 +314,7 @@ int sclp_service_call(CPUS390XState *env, uint64_t sccb, uint32_t code)
>>          goto out_write;
>>      }
>>  
>> -    if ((sccb + be16_to_cpu(work_sccb.h.length)) > ((sccb & PAGE_MASK) + PAGE_SIZE)) {
>> -        work_sccb.h.response_code = cpu_to_be16(SCLP_RC_SCCB_BOUNDARY_VIOLATION);
>> +    if (!sccb_has_valid_boundary(sccb, code, &work_sccb.h)) {
>>          goto out_write;
>>      }
>>  
>>
> 
> 


-- 
--
Regards,
Collin

Stay safe and stay healthy