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

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Date: Mon, 26 Feb 2024 18:08:14 -0800
From: Kees Cook <keescook@...omium.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: rafael@...nel.org, daniel.lezcano@...aro.org, rui.zhang@...el.com,
	lukasz.luba@....com, 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 Mon, Feb 26, 2024 at 05:54:58PM -0700, 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

Yay, the "better details" output is working. :)

>   [    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));

Looks good to me; thanks for catching this!

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook