[PATCH AUTOSEL 6.1 41/42] fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

Sasha Levin
2024-10-04T18:28:36Z

From: Qianqiang Liu <qianqiang.liu@163.com>

[ Upstream commit 5b97eebcce1b4f3f07a71f635d6aa3af96c236e7 ]

syzbot has found a NULL pointer dereference bug in fbcon.
Here is the simplified C reproducer:

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

int main()
{
struct fb_con2fbmap con2fb;
struct param param;

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

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

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

int fd1 = open("/dev/tty1", O_RDWR, 0);
ioctl(fd1, TIOCLINUX, &param);

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

return 0;
}

After calling ioctl(fd1, TIOCLINUX, &param), the subsequent ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb)
causes the kernel to follow a different execution path:

set_con2fb_map
-> con2fb_init_display
-> fbcon_set_disp
-> redraw_screen
-> hide_cursor
-> clear_selection
-> highlight
-> invert_screen
-> do_update_region
-> fbcon_putcs
-> ops->putcs

Since ops->putcs is a NULL pointer, this leads to a kernel panic.
To prevent this, we need to call set_blitting_type() within set_con2fb_map()
to properly initialize ops->putcs.

Reported-by: syzbot+3d613ae53c031502687a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3d613ae53c031502687a
Tested-by: syzbot+3d613ae53c031502687a@syzkaller.appspotmail.com
Signed-off-by: Qianqiang Liu <qianqiang.liu@163.com>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/video/fbdev/core/fbcon.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 14498a0d13e0b..e6640edec155e 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -847,6 +847,8 @@ static int set_con2fb_map(int unit, int newidx, int user)
return err;

fbcon_add_cursor_work(info);
+ } else if (vc) {
+ set_blitting_type(vc, info);
}

con2fb_map[unit] = newidx;
--
2.43.0