linux-kernel - Re: [PATCH v3] clk: zynq: Prevent null pointer dereference caused by kmalloc failure

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Date: Fri, 1 Mar 2024 15:24:35 +0800 (GMT+08:00)
From: duoming@....edu.cn
To: "Michal Simek" <michal.simek@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-clk@...r.kernel.org, mturquette@...libre.com, sboyd@...nel.org
Subject: Re: [PATCH v3] clk: zynq: Prevent null pointer dereference caused
 by kmalloc failure

On Fri, 1 Mar 2024 08:05:17 +0100 Michal Simek wrote:
> > The kmalloc() in zynq_clk_setup() will return null if the
> > physical memory has run out. As a result, if we use snprintf()
> > to write data to the null address, the null pointer dereference
> > bug will happen.
> > 
> > This patch uses a stack variable to replace the kmalloc().
> > 
> > Fixes: 0ee52b157b8e ("clk: zynq: Add clock controller driver")
> > Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> > ---
> > Changes in v3:
> >    - Put stack variable in the head of the function and remove variable tmp.
> > 
> >   drivers/clk/zynq/clkc.c | 7 ++-----
> >   1 file changed, 2 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/clk/zynq/clkc.c b/drivers/clk/zynq/clkc.c
> > index 7bdeaff2bfd..45479653c3b 100644
> > --- a/drivers/clk/zynq/clkc.c
> > +++ b/drivers/clk/zynq/clkc.c
> > @@ -215,7 +215,7 @@ static void __init zynq_clk_setup(struct device_node *np)
> >   	int i;
> >   	u32 tmp;
> >   	int ret;
> > -	char *clk_name;
> > +	char clk_name[16];
> >   	unsigned int fclk_enable = 0;
> >   	const char *clk_output_name[clk_max];
> >   	const char *cpu_parents[4];
> > @@ -426,12 +426,10 @@ static void __init zynq_clk_setup(struct device_node *np)
> >   			"gem1_emio_mux", CLK_SET_RATE_PARENT,
> >   			SLCR_GEM1_CLK_CTRL, 0, 0, &gem1clk_lock);
> >   
> > -	tmp = strlen("mio_clk_00x");
> > -	clk_name = kmalloc(tmp, GFP_KERNEL);
> >   	for (i = 0; i < NUM_MIO_PINS; i++) {
> >   		int idx;
> >   
> > -		snprintf(clk_name, tmp, "mio_clk_%2.2d", i);
> > +		snprintf(clk_name, 16, "mio_clk_%2.2d", i);
> 
> I would prefer to use macro/define if that values has to match and have connection.

The origin code "clk_name = kmalloc(tmp, GFP_KERNEL)" allocates 16 bytes, so I set
the size of stack variable clk_name to 16 bytes. 

Do you think if we use "#define CLK_NAME_LEN 16" is better?

Best regards,
Duoming Zhou