linux-kernel - Re: [PATCH 2/2] mfd: omap-usb-tll: use struct

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Date: Wed, 26 Jun 2024 19:57:16 +0200
From: Jann Horn <jannh@...gle.com>
To: Javier Carrasco <javier.carrasco.cruz@...il.com>, Markus Elfring <Markus.Elfring@....de>, 
	Ladislav Michl <ladis@...ux-mips.org>, Roger Quadros <rogerq@...nel.org>, Lee Jones <lee@...nel.org>
Cc: Tony Lindgren <tony@...mide.com>, Kees Cook <kees@...nel.org>, 
	"Gustavo A. R. Silva" <gustavoars@...nel.org>, linux-omap@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/2] mfd: omap-usb-tll: use struct_size to allocate tll

On Thu, Jun 20, 2024 at 11:23 PM Javier Carrasco
<javier.carrasco.cruz@...il.com> wrote:
>
> Use the struct_size macro to calculate the size of the tll, which
> includes a trailing flexible array.
>
> Signed-off-by: Javier Carrasco <javier.carrasco.cruz@...il.com>
>
> ---
> The memory allocation used to be carried out in two steps:
>
> tll = devm_kzalloc(dev, sizeof(struct usbtll_omap), GFP_KERNEL);
> tll->ch_clk = devm_kzalloc(dev, sizeof(struct clk *) * tll->nch,
>                            GFP_KERNEL);
>
> Until commit 16c2004d9e4d ("mfd: omap-usb-tll: Allocate driver data at once")
> turned that into the current allocation:
>
> tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
>                    GFP_KERNEL);
>
> That has surprised me at first glance because I would have expected
> sizeof(tll->ch_clk[nch]) to return the size of a single pointer, not
> being equivalent to 'sizeof(struct clk *) * nch'.
>
> I might be missing/misunderstanding something here because the commit
> is not new, and the error should be noticeable. Moreover, I don't have
> real hardware to test it. Hence why I didn't mark this patch as a fix.
>
> I would be pleased to get feedback about this (why it is right as it is,
> or if that is actually a bug).

I might be a good idea to ask the people who wrote / accepted the
patch that seems to have introduced the bug? I've CC'ed them.

The kernel slab allocator enforces a minimum alignment of
KMALLOC_MIN_SIZE, which on MIPS seems to be based on the size of L1
cache lines or something like that? This allocator alignment might be
enough to prevent the bug from actually causing an observable effect
in many configurations - that could explain why it went undetected,
assuming that nobody tested this in a KASAN build.



> ---
>  drivers/mfd/omap-usb-tll.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/mfd/omap-usb-tll.c b/drivers/mfd/omap-usb-tll.c
> index a091e5b0f21d..5f25ac514ff2 100644
> --- a/drivers/mfd/omap-usb-tll.c
> +++ b/drivers/mfd/omap-usb-tll.c
> @@ -230,8 +230,7 @@ static int usbtll_omap_probe(struct platform_device *pdev)
>                 break;
>         }
>
> -       tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
> -                          GFP_KERNEL);
> +       tll = devm_kzalloc(dev, struct_size(tll, ch_clk, nch), GFP_KERNEL);
>         if (!tll) {
>                 pm_runtime_put_sync(dev);
>                 pm_runtime_disable(dev);
>
> --
> 2.40.1
>
>