linux-kernel - Re: [PATCH v2] clk: zynq: Prevent null pointer dereference caused by kmalloc failure

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Message-ID: <365f9a90.55b0.18df5394935.Coremail.duoming@zju.edu.cn>
Date: Thu, 29 Feb 2024 22:16:28 +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, sboyd@...nel.org, mturquette@...libre.com
Subject: Re: [PATCH v2] clk: zynq: Prevent null pointer dereference caused
 by kmalloc failure

On Thu, 29 Feb 2024 13:45:54 +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 adds 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 v2:
> >    - Use stack variable to replace kmalloc().
> > 
> >   drivers/clk/zynq/clkc.c | 3 +--
> >   1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/drivers/clk/zynq/clkc.c b/drivers/clk/zynq/clkc.c
> > index 7bdeaff2bfd..e4c4c9adf79 100644
> > --- a/drivers/clk/zynq/clkc.c
> > +++ b/drivers/clk/zynq/clkc.c
> > @@ -427,7 +427,7 @@ static void __init zynq_clk_setup(struct device_node *np)
> >   			SLCR_GEM1_CLK_CTRL, 0, 0, &gem1clk_lock);
> >   
> >   	tmp = strlen("mio_clk_00x");
> > -	clk_name = kmalloc(tmp, GFP_KERNEL);
> > +	char clk_name[tmp];
> 
> I know that Stephen asked for it but variable with variable length in the middle 
> of code doesn't look good or useful.
> I would allocate rather bigger array on stack with size bigger than max length 
> which will use it.

The length of "mio_clk_00x" is 11 bytes, and the kernel will alloc 16 bytes to it.
I use a local variable whose size is 16 bytes to replace it. The detail is shown
below:

diff --git a/drivers/clk/zynq/clkc.c b/drivers/clk/zynq/clkc.c
index 7bdeaff2bfd..81d530e3357 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];
@@ -427,7 +427,6 @@ static void __init zynq_clk_setup(struct device_node *np)
                        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;

@@ -439,7 +438,6 @@ static void __init zynq_clk_setup(struct device_node *np)
                else
                        can_mio_mux_parents[i] = dummy_nm;
        }
-       kfree(clk_name);
        clk_register_mux(NULL, "can_mux", periph_parents, 4,
                        CLK_SET_RATE_NO_REPARENT, SLCR_CAN_CLK_CTRL, 4, 2, 0,
                        &canclk_lock);

Do you think the above is better?

Best regards,
Duoming Zhou