linux-hardening - Re: [PATCH] thermal: core: Move initial num

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Date: Tue, 27 Feb 2024 16:37:36 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, rui.zhang@...el.com,
 lukasz.luba@....com, keescook@...omium.org, gustavoars@...nel.org,
 morbo@...gle.com, justinstitt@...gle.com, stanislaw.gruszka@...ux.intel.com,
 linux-pm@...r.kernel.org, linux-hardening@...r.kernel.org,
 llvm@...ts.linux.dev, patches@...ts.linux.dev
Subject: Re: [PATCH] thermal: core: Move initial num_trips assignment before
 memcpy()

On 27/02/2024 12:09, Rafael J. Wysocki wrote:
> On Tue, Feb 27, 2024 at 11:14 AM Daniel Lezcano
> <daniel.lezcano@...aro.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@...nel.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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