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Date: Wed, 11 Oct 2023 14:38:59 -0700
From: Kees Cook <keescook@...omium.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Dinh Nguyen <dinguyen@...nel.org>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2][next] clk: socfpga: Fix undefined behavior bug in
 struct stratix10_clock_data

On Wed, Oct 11, 2023 at 03:34:03PM -0600, Gustavo A. R. Silva wrote:
> `struct clk_hw_onecell_data` is a flexible structure, which means that
> it contains flexible-array member at the bottom, in this case array
> `hws`:
> 
> include/linux/clk-provider.h:
> 1380 struct clk_hw_onecell_data {
> 1381         unsigned int num;
> 1382         struct clk_hw *hws[] __counted_by(num);
> 1383 };
> 
> This could potentially lead to an overwrite of the objects following
> `clk_data` in `struct stratix10_clock_data`, in this case
> `void __iomem *base;` at run-time:
> 
> drivers/clk/socfpga/stratix10-clk.h:
>   9 struct stratix10_clock_data {
>  10         struct clk_hw_onecell_data      clk_data;
>  11         void __iomem            *base;
>  12 };
> 
> There are currently three different places where memory is allocated for
> `struct stratix10_clock_data`, including the flex-array `hws` in
> `struct clk_hw_onecell_data`:
> 
> drivers/clk/socfpga/clk-agilex.c:
> 469         clk_data = devm_kzalloc(dev, struct_size(clk_data, clk_data.hws,
> 470                                 num_clks), GFP_KERNEL);
> 
> drivers/clk/socfpga/clk-agilex.c:
> 509         clk_data = devm_kzalloc(dev, struct_size(clk_data, clk_data.hws,
> 510                                 num_clks), GFP_KERNEL);
> 
> drivers/clk/socfpga/clk-s10.c:
> 400         clk_data = devm_kzalloc(dev, struct_size(clk_data, clk_data.hws,
> 401                                                  num_clks), GFP_KERNEL);
> 
> I'll use just one of them to describe the issue. See below.
> 
> Notice that a total of 440 bytes are allocated for flexible-array member
> `hws` at line 469:
> 
> include/dt-bindings/clock/agilex-clock.h:
>  70 #define AGILEX_NUM_CLKS	55
> 
> drivers/clk/socfpga/clk-agilex.c:
> 459         struct stratix10_clock_data *clk_data;
> 460         void __iomem *base;
> ...
> 466
> 467         num_clks = AGILEX_NUM_CLKS;
> 468
> 469         clk_data = devm_kzalloc(dev, struct_size(clk_data, clk_data.hws,
> 470                                 num_clks), GFP_KERNEL);
> 
> `struct_size(clk_data, clk_data.hws, num_clks)`	above translates to
> sizeof(struct stratix10_clock_data) + sizeof(struct clk_hw *) * 55 ==
> 16 + 8 * 55 == 16 + 440
> 		    ^^^
> 		     |
> 	allocated bytes for flex-array `hws`
> 
> 474         for (i = 0; i < num_clks; i++)
> 475                 clk_data->clk_data.hws[i] = ERR_PTR(-ENOENT);
> 476
> 477         clk_data->base = base;
> 
> and then some data is written into both `hws` and `base` objects.
> 
> Fix this by placing the declaration of object `clk_data` at the end of
> `struct stratix10_clock_data`. Also, add a comment to make it clear
> that this object must always be last in the structure.
> 
> Fixes: ba7e258425ac ("clk: socfpga: Convert to s10/agilex/n5x to use clk_hw")
> Cc: stable@...r.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Nice find!

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

-- 
Kees Cook

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