[PATCH] fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

[PATCH] fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

[Qianqiang Liu]

syzbot has found a NULL pointer dereference bug in fbcon [1].

This issue is caused by ops->putcs being a NULL pointer.
We need to check the pointer before using it.

[1] https://syzkaller.appspot.com/bug?extid=3d613ae53c031502687a

Cc: stable@vger.kernel.org
Reported-and-tested-by: syzbot+3d613ae53c031502687a@syzkaller.appspotmail.com
Signed-off-by: Qianqiang Liu <qianqiang.liu@163.com>
---
 drivers/video/fbdev/core/fbcon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 3f7333dca508..96c1262cc981 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -1284,7 +1284,7 @@ static void fbcon_putcs(struct vc_data *vc, const u16 *s, unsigned int count,
 	struct fbcon_display *p = &fb_display[vc->vc_num];
 	struct fbcon_ops *ops = info->fbcon_par;
 
-	if (!fbcon_is_inactive(vc, info))
+	if (!fbcon_is_inactive(vc, info) && ops->putcs)
 		ops->putcs(vc, info, s, count, real_y(p, ypos), xpos,
 			   get_color(vc, info, scr_readw(s), 1),
 			   get_color(vc, info, scr_readw(s), 0));
-- 
2.39.2

Re: [PATCH] fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

[Helge Deller]

On 9/16/24 03:10, Qianqiang Liu wrote:
> syzbot has found a NULL pointer dereference bug in fbcon [1].
>
> This issue is caused by ops->putcs being a NULL pointer.
> We need to check the pointer before using it.
>
> [1] https://syzkaller.appspot.com/bug?extid=3d613ae53c031502687a
>
> Cc: stable@vger.kernel.org
> Reported-and-tested-by: syzbot+3d613ae53c031502687a@syzkaller.appspotmail.com
> Signed-off-by: Qianqiang Liu <qianqiang.liu@163.com>
> ---
>   drivers/video/fbdev/core/fbcon.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 3f7333dca508..96c1262cc981 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -1284,7 +1284,7 @@ static void fbcon_putcs(struct vc_data *vc, const u16 *s, unsigned int count,
>   	struct fbcon_display *p = &fb_display[vc->vc_num];
>   	struct fbcon_ops *ops = info->fbcon_par;
>
> -	if (!fbcon_is_inactive(vc, info))
> +	if (!fbcon_is_inactive(vc, info) && ops->putcs)

I think this patch just hides the real problem.
How could putcs have become NULL ?

Helge

>   		ops->putcs(vc, info, s, count, real_y(p, ypos), xpos,
>   			   get_color(vc, info, scr_readw(s), 1),
>   			   get_color(vc, info, scr_readw(s), 0));

[PATCH] fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

[Qianqiang Liu]


> I think this patch just hides the real problem.
> How could putcs have become NULL ?
> 
> Helge

Oh, you are right!
I will figure it out.

Best,
Qianqiang Liu

Re: [PATCH] fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

[Qianqiang Liu]

Hi, 

I simplified the C reproducer as follows:

#include <stdint.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <linux/tiocl.h>
#include <sys/ioctl.h>

struct param {
        uint8_t type;
        struct tiocl_selection ts;
};

int main()
{
        write(1, "executing program\n", sizeof("executing program\n") - 1);

        int fd = open("/dev/fb1", 0, 0);

        struct fb_con2fbmap con2fb;
        con2fb.console = 0x19;
        con2fb.framebuffer = 0;
        ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb);

        int fd1 = open("/dev/tty1", O_RDWR, 0);

        struct param param;
        param.type = 2;
        param.ts.xs = 0;
        param.ts.ys = 0;
        param.ts.xe = 0;
        param.ts.ye = 0;
        param.ts.sel_mode = 0;

        ioctl(fd1, TIOCLINUX, &param);

        con2fb.console = 1;
        con2fb.framebuffer = 0;
        ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb);

        return 0;
}

But I still need time to debug the kernel code..

-- 
Best,
Qianqiang Liu