linux-kernel - Re: [PATCH v2] vt: don't use kmalloc() for the unicode screen buffer

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Date:   Mon, 30 Mar 2020 21:07:59 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     gregkh@...uxfoundation.org, Chen Wandun <chenwandun@...wei.com>,
        Adam Borowski <kilobyte@...band.pl>, jslaby@...e.com,
        daniel.vetter@...ll.ch, b.zolnierkie@...sung.com, lukas@...ner.de,
        ghalat@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vt: don't use kmalloc() for the unicode screen buffer

Hi Nicolas

On Sat, Mar 28, 2020 at 10:25:11PM -0400, Nicolas Pitre wrote:
> Even if the actual screen size is bounded in vc_do_resize(), the unicode 
> buffer is still a little more than twice the size of the glyph buffer
> and may exceed MAX_ORDER down the kmalloc() path. This can be triggered
> from user space.
> 
> Since there is no point having a physically contiguous buffer here, 
> let's avoid the above issue as well as reducing pressure on high order
> allocations by using vmalloc() instead.
> 
> Signed-off-by: Nicolas Pitre <nico@...xnic.net>
> Cc: <stable@...r.kernel.org>
> 
> ---
> 
> Changes since v1:
> 
> - Added missing include, found by kbuild test robot.
>   Strange that my own build doesn't complain.

When I did the drmP.h removal vmalloc was one of the header files
that turned up missing in many cases - but only for some architectures.
I learned to include alpha in the build.
If it survived building for alpha then I had fixed the majority
of the issues related to random inherited includes.

The patch itself looks good.

Acked-by: Sam Ravnborg <sam@...nborg.org>

> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 15d2769805..d9eb5661e9 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -81,6 +81,7 @@
>  #include <linux/errno.h>
>  #include <linux/kd.h>
>  #include <linux/slab.h>
> +#include <linux/vmalloc.h>
>  #include <linux/major.h>
>  #include <linux/mm.h>
>  #include <linux/console.h>
> @@ -350,7 +351,7 @@ static struct uni_screen *vc_uniscr_alloc(unsigned int cols, unsigned int rows)
>  	/* allocate everything in one go */
>  	memsize = cols * rows * sizeof(char32_t);
>  	memsize += rows * sizeof(char32_t *);
> -	p = kmalloc(memsize, GFP_KERNEL);
> +	p = vmalloc(memsize);
>  	if (!p)
>  		return NULL;
>  
> @@ -366,7 +367,7 @@ static struct uni_screen *vc_uniscr_alloc(unsigned int cols, unsigned int rows)
>  
>  static void vc_uniscr_set(struct vc_data *vc, struct uni_screen *new_uniscr)
>  {
> -	kfree(vc->vc_uni_screen);
> +	vfree(vc->vc_uni_screen);
>  	vc->vc_uni_screen = new_uniscr;
>  }
>