linux-hardening - Re: [PATCH] gcc-plugins: Undefine LATENT_ENTROPY

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Message-ID: <YvuKKngQ/l24+7rH@yury-laptop>
Date:   Tue, 16 Aug 2022 05:14:34 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Andrew Donnellan <ajd@...ux.ibm.com>
Cc:     linux-hardening@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Emese Revfy <re.emese@...il.com>,
        Erhard Furtner <erhard_f@...lbox.org>
Subject: Re: [PATCH] gcc-plugins: Undefine LATENT_ENTROPY_PLUGIN when plugin
 disabled for a file

On Tue, Aug 16, 2022 at 03:17:20PM +1000, Andrew Donnellan wrote:
> Commit 36d4b36b6959 ("lib/nodemask: inline next_node_in() and
> node_random()") refactored some code by moving node_random() from
> lib/nodemask.c to include/linux/nodemask.h, thus requiring nodemask.h to
> include random.h, which conditionally defines add_latent_entropy()
> depending on whether the macro LATENT_ENTROPY_PLUGIN is defined.
> 
> This broke the build on powerpc, where nodemask.h is indirectly included
> in arch/powerpc/kernel/prom_init.c, part of the early boot machinery that
> is excluded from the latent entropy plugin using
> DISABLE_LATENT_ENTROPY_PLUGIN. It turns out that while we add a gcc flag
> to disable the actual plugin, we don't undefine LATENT_ENTROPY_PLUGIN.
> 
> This leads to the following:
> 
>     CC      arch/powerpc/kernel/prom_init.o
>   In file included from ./include/linux/nodemask.h:97,
>                    from ./include/linux/mmzone.h:17,
>                    from ./include/linux/gfp.h:7,
>                    from ./include/linux/xarray.h:15,

As a side note, xarray can go with gfp_types.h instead of gfp.h

>                    from ./include/linux/radix-tree.h:21,
>                    from ./include/linux/idr.h:15,
>                    from ./include/linux/kernfs.h:12,
>                    from ./include/linux/sysfs.h:16,
>                    from ./include/linux/kobject.h:20,
>                    from ./include/linux/pci.h:35,
>                    from arch/powerpc/kernel/prom_init.c:24:
>   ./include/linux/random.h: In function 'add_latent_entropy':
>   ./include/linux/random.h:25:46: error: 'latent_entropy' undeclared (first use in this function); did you mean 'add_latent_entropy'?
>      25 |         add_device_randomness((const void *)&latent_entropy, sizeof(latent_entropy));
>         |                                              ^~~~~~~~~~~~~~
>         |                                              add_latent_entropy
>   ./include/linux/random.h:25:46: note: each undeclared identifier is reported only once for each function it appears in
>   make[2]: *** [scripts/Makefile.build:249: arch/powerpc/kernel/prom_init.o] Fehler 1
>   make[1]: *** [scripts/Makefile.build:465: arch/powerpc/kernel] Fehler 2
>   make: *** [Makefile:1855: arch/powerpc] Error 2
> 
> Change the DISABLE_LATENT_ENTROPY_PLUGIN flags to undefine
> LATENT_ENTROPY_PLUGIN for files where the plugin is disabled.
> 
> Cc: Yury Norov <yury.norov@...il.com>
> Cc: Emese Revfy <re.emese@...il.com>
> Fixes: 36d4b36b6959 ("lib/nodemask: inline next_node_in() and node_random()")

I think it rather fixes 38addce8b600ca33 ("gcc-plugins: Add
latent_entropy plugin").

For the rest,
Reviewed-by: Yury Norov <yury.norov@...il.com>

> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216367
> Link: https://lore.kernel.org/linuxppc-dev/alpine.DEB.2.22.394.2208152006320.289321@ramsan.of.borg/
> Reported-by: Erhard Furtner <erhard_f@...lbox.org>
> Signed-off-by: Andrew Donnellan <ajd@...ux.ibm.com>
> ---
>  scripts/Makefile.gcc-plugins | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
> index 692d64a70542..e4deaf5fa571 100644
> --- a/scripts/Makefile.gcc-plugins
> +++ b/scripts/Makefile.gcc-plugins
> @@ -4,7 +4,7 @@ gcc-plugin-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY)	+= latent_entropy_plugin.so
>  gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY)		\
>  		+= -DLATENT_ENTROPY_PLUGIN
>  ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
> -    DISABLE_LATENT_ENTROPY_PLUGIN += -fplugin-arg-latent_entropy_plugin-disable
> +    DISABLE_LATENT_ENTROPY_PLUGIN += -fplugin-arg-latent_entropy_plugin-disable -ULATENT_ENTROPY_PLUGIN
>  endif
>  export DISABLE_LATENT_ENTROPY_PLUGIN
>  
> -- 
> 2.30.2