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

Daniel Lezcano
2024-02-27T15:37:39Z

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

then:

memcpy(tz->trips, trips, num_trips * sizeof(*trips));

memcpy(tz->trips, trips, num_trips * 24 * num_trips);

==> memory size = 24 * num_trips^2

(2) if the compiler does not support it:

sizeof(*trips) == 24 bytes

then:

memcpy(tz->trips, trips, num_trips * sizeof(*trips));

memcpy(tz->trips, trips, num_trips * 24);

==> memory size = 24 * num_trips

Or did I misunderstand __counted_by ?

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