linux-kernel - Re: [PATCH] drm/i915: Fix CFI violation with show_dynamic

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Date:   Thu, 12 May 2022 14:30:17 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <nathan@...nel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
CC:     Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev
Subject: Re: [PATCH] drm/i915: Fix CFI violation with show_dynamic_id()



On May 12, 2022 2:17:04 PM PDT, Nathan Chancellor <nathan@...nel.org> wrote:
>When an attribute group is created with sysfs_create_group(), the
>->sysfs_ops() callback is set to kobj_sysfs_ops, which sets the ->show()
>callback to kobj_attr_show(). kobj_attr_show() uses container_of() to
>get the ->show() callback from the attribute it was passed, meaning the
>->show() callback needs to be the same type as the ->show() callback in
>'struct kobj_attribute'.
>
>However, show_dynamic_id() has the type of the ->show() callback in
>'struct device_attribute', which causes a CFI violation when opening the
>'id' sysfs node under drm/card0/metrics. This happens to work because
>the layout of 'struct kobj_attribute' and 'struct device_attribute' are
>the same, so the container_of() cast happens to allow the ->show()
>callback to still work.
>
>Change the type of show_dynamic_id() to match the ->show() callback in
>'struct kobj_attributes' and update the type of sysfs_metric_id to
>match, which resolves the CFI violation.
>
>Fixes: f89823c21224 ("drm/i915/perf: Implement I915_PERF_ADD/REMOVE_CONFIG interface")
>Signed-off-by: Nathan Chancellor <nathan@...nel.org>

This matches my own investigation into the error. Thanks for putting the patch together! :)

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


-- 
Kees Cook