On Thu, Jan 17, 2019 at 09:32:55AM +0800, Jia Zhang wrote:
>
>
> 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@xxxxxxxxxxxxxxxxx>
> >
> > 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.
OK, got it now. I think I will apply this! Will take a while because
of https://lkml.org/lkml/2019/1/18/485. Will not apply new patches
before that is rooted.
/Jarkko
