Re: [PATCH 1/2] tpm/eventlog/tpm1: Simplify walking over *pos measurements

[Jia Zhang <zhang.jia@linux.alibaba.com>]

On 2019/1/17 上午6:09, Jarkko Sakkinen wrote:
> Please use "tpm:" tag for commits, not "tpm/eventlog/tpm1".
> 
> On Fri, Jan 11, 2019 at 04:59:32PM +0800, Jia Zhang wrote:
>> The responsibility of tpm1_bios_measurements_start() is to walk
>> over the first *pos measurements, ensuring the skipped and
>> to-be-read measurements are not out-of-boundary.
>>
>> Current logic is complicated a bit. Just employ a do-while loop
>> with necessary sanity check, and then get the goal.
>>
>> Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>
> 
> What does this fix? Even if the current logic is "complicated", it is
> still a pretty simple functiion.


OK. Let me point out the fix part. Here is the original implementation:

 87         /* read over *pos measurements */
 88         for (i = 0; i < *pos; i++) {
 89                 event = addr;
 90
 91                 converted_event_size =
 92                     do_endian_conversion(event->event_size);
 93                 converted_event_type =
 94                     do_endian_conversion(event->event_type);
 95
 96                 if ((addr + sizeof(struct tcpa_event)) < limit) {
 97                         if ((converted_event_type == 0) &&
 98                             (converted_event_size == 0))
 99                                 return NULL;
100                         addr += (sizeof(struct tcpa_event) +
101                                  converted_event_size);
102                 }
103         }

The problem (just ignore all off-by-1 issues) is that accessing to
event_size and event_type is not pre-checked carefully. In the latter
part of tpm1_bios_measurements_start() and
tpm1_bios_measurements_next(), there is a fixed patter to do the sanity
check like this:

136         /* now check if current entry is valid */
137         if ((v + sizeof(struct tcpa_event)) >= limit)
138                 return NULL;

So if we simply change this read-over chunk with sanity check like this:

        /* read over *pos measurements */
        for (i = 0; i < *pos; i++) {
                event = addr;

                if ((addr + sizeof(struct tcpa_event)) >= limit)
                        return NULL;

                converted_event_size =
                    do_endian_conversion(event->event_size);
                converted_event_type =
                    do_endian_conversion(event->event_type);

                if ((converted_event_type == 0) &&
                    (converted_event_size == 0))
                        return NULL;
                addr += (sizeof(struct tcpa_event) +
                         converted_event_size);
        }

We will get two highly similar code chunks in
tpm1_bios_measurements_start(). Here is the latter part:

106         /* now check if current entry is valid */
107         if ((addr + sizeof(struct tcpa_event)) >= limit)
108                 return NULL;
109
110         event = addr;
111
112         converted_event_size = do_endian_conversion(event->event_size);
113         converted_event_type = do_endian_conversion(event->event_type);
114
115         if (((converted_event_type == 0) && (converted_event_size == 0))
116             || ((addr + sizeof(struct tcpa_event) +
converted_event_size)
117                 >= limit))
118                 return NULL;
119
120         return addr;

So using a do while logic can simply merge them together and thus simply
and optimize the logic of walking over *pos measurements.

Sorry I admit my initial motivation is to fix up the sanity check
problem. If you would like to accept the optimization part, I will split
this patch.

Jia

> 
> Applying clean ups for fun has the side-effect of making backporting
> more difficult. And swapping implementation randomly has the side-effect
> of potentially introducing regressions. The current code might be messy
> but it is still field tested.
> 
> I'm sorry but I have to reject this patch.
> 
> /Jarkko
>