Re: [PATCH] thermal: core: Move initial num_trips assignment before memcpy()

Daniel Lezcano
2024-02-27T16:47:47Z

On 27/02/2024 17:26, Kees Cook wrote:
> On Tue, Feb 27, 2024 at 04:37:36PM +0100, Daniel Lezcano wrote:
>> On 27/02/2024 12:09, Rafael J. Wysocki wrote:
>>> On Tue, Feb 27, 2024 at 11:14 AM Daniel Lezcano
>>> <daniel.lezcano@linaro.org> wrote:
>>>>
>>>> On 27/02/2024 01:54, Nathan Chancellor wrote:
>>>>> When booting a CONFIG_FORTIFY_SOURCE=y kernel compiled with a toolchain
>>>>> that supports __counted_by() (such as clang-18 and newer), there is a
>>>>> panic on boot:
>>>>>
>>>>> [ 2.913770] memcpy: detected buffer overflow: 72 byte write of buffer size 0
>>>>> [ 2.920834] WARNING: CPU: 2 PID: 1 at lib/string_helpers.c:1027 __fortify_report+0x5c/0x74
>>>>> ...
>>>>> [ 3.039208] Call trace:
>>>>> [ 3.041643] __fortify_report+0x5c/0x74
>>>>> [ 3.045469] __fortify_panic+0x18/0x20
>>>>> [ 3.049209] thermal_zone_device_register_with_trips+0x4c8/0x4f8
>>>>>
>>>>> This panic occurs because trips is counted by num_trips but num_trips is
>>>>> assigned after the call to memcpy(), so the fortify checks think the
>>>>> buffer size is zero because tz was allocated with kzalloc().
>>>>>
>>>>> Move the num_trips assignment before the memcpy() to resolve the panic
>>>>> and ensure that the fortify checks work properly.
>>>>>
>>>>> Fixes: 9b0a62758665 ("thermal: core: Store zone trips table in struct thermal_zone_device")
>>>>> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
>>>>> ---
>>>>> drivers/thermal/thermal_core.c | 2 +-
>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c
>>>>> index bb21f78b4bfa..1eabc8ebe27d 100644
>>>>> --- a/drivers/thermal/thermal_core.c
>>>>> +++ b/drivers/thermal/thermal_core.c
>>>>> @@ -1354,8 +1354,8 @@ thermal_zone_device_register_with_trips(const char *type,
>>>>>
>>>>> tz->device.class = thermal_class;
>>>>> tz->devdata = devdata;
>>>>> - memcpy(tz->trips, trips, num_trips * sizeof(*trips));
>>>>> tz->num_trips = num_trips;
>>>>> + memcpy(tz->trips, trips, num_trips * sizeof(*trips));
>>>>
>>>> IIUC, clang-18 is used and supports __counted_by().
>>>>
>>>> Is it possible sizeof(*trips) returns already the real trips array size
>>>> and we are multiplying it again by num_trips ?
>>>>
>>>> While with an older compiler, __counted_by() does nothing and we have to
>>>> multiply by num_trips ?
>>>>
>>>> IOW, the array size arithmetic is different depending if we have
>>>> _counted_by supported or not ?
>>>
>>> IIUC it is just the instrumentation using the current value of
>>> tz->num_trips (which is 0 before the initialization).
>>
>> Right, but I am wondering if
>>
>> memcpy(tz->trips, trips, num_trips * sizeof(*trips));
>>
>> is still correct with __counted_by because:
>>
>> (1) if the compiler supports it:
>>
>> sizeof(*trips) == 24 bytes * num_trips
>
> I think you're misunderstanding. The above sizeof() only evaluates a
> single instance -- it has no idea how many more there may be.
> Specifically:
>
> sizeof(*trips) == sizeof(struct thermal_trip)
>
>> then:
>>
>> memcpy(tz->trips, trips, num_trips * sizeof(*trips));
>>
>> memcpy(tz->trips, trips, num_trips * 24 * num_trips);
>>
>> ==> memory size = 24 * num_trips^2
>
> It's not counted twice. Under CONFIG_FORTIFY_SOURCE=y, memcpy is a macro
> that expands to a checking routine (see include/linux/fortify-string.h),
> which is using __builtin_dynamic_object_size() to determine the
> available size of the destination buffer (tz->trips). Specifically:
>
> __builtin_dynamic_object_size(tz->trips)
>
> When __bdos evaluates a flexible array (i.e. tz->trips), it will see the
> associated 'counted_by' attribute, and go look up the value of the
> designated struct member (tz->num_trips). It then calculates:
>
> sizeof(*tz->trips) /* a single instance */
> *
> tz->num_trips

Ok my misunderstanding was I thought sizeof() was calling _bdos under
the hood, so when calling sizeof(flex_array), it was returning the
computed size inferring from the __counted_by field.

> Before the patch, tz->num_trips is 0, so the destination buffer size
> appears to be of size 0 bytes. After the patch, it contains the
> same value as the "num_trips" function argument, so the destination
> buffer appears to be the matching size of "num_trips * sizeof(struct
> thermal_trip)".
>
> Hopefully that helps! If not, I can try again. :)

It is ok thanks for the clarification

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