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

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Date: Tue, 27 Feb 2024 11:14:04 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Nathan Chancellor <nathan@...nel.org>, rafael@...nel.org
Cc: 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 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 ?



>   	thermal_set_delay_jiffies(&tz->passive_delay_jiffies, passive_delay);
>   	thermal_set_delay_jiffies(&tz->polling_delay_jiffies, polling_delay);
> 
> ---
> base-commit: a85739c8c6894c3b9ff860e79e91db44cb59bd63
> change-id: 20240226-thermal-fix-fortify-panic-num_trips-5f94094fb963
> 
> Best regards,

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