[PATCH] fbcon: Avoid showing softcursor if font not available

[Sam Ravnborg]

Hi Helge,

On Sat, May 27, 2023 at 11:19:02AM +0200, Helge Deller wrote:
> syzbot repored this bug in the softcursor code:
> 
> BUG: KASAN: null-ptr-deref in soft_cursor+0x384/0x6b4 drivers/video/fbdev/core/softcursor.c:70
> Read of size 16 at addr 0000000000000200 by task kworker/u4:1/12
> 
> CPU: 0 PID: 12 Comm: kworker/u4:1 Not tainted 6.4.0-rc3-syzkaller-geb0f1697d729 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/28/2023
> Workqueue: events_power_efficient fb_flashcursor
> Call trace:
>  dump_backtrace+0x1b8/0x1e4 arch/arm64/kernel/stacktrace.c:233
>  show_stack+0x2c/0x44 arch/arm64/kernel/stacktrace.c:240
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0xd0/0x124 lib/dump_stack.c:106
>  print_report+0xe4/0x514 mm/kasan/report.c:465
>  kasan_report+0xd4/0x130 mm/kasan/report.c:572
>  kasan_check_range+0x264/0x2a4 mm/kasan/generic.c:187
>  __asan_memcpy+0x3c/0x84 mm/kasan/shadow.c:105
>  soft_cursor+0x384/0x6b4 drivers/video/fbdev/core/softcursor.c:70
>  bit_cursor+0x113c/0x1a64 drivers/video/fbdev/core/bitblit.c:377
>  fb_flashcursor+0x35c/0x54c drivers/video/fbdev/core/fbcon.c:380
>  process_one_work+0x788/0x12d4 kernel/workqueue.c:2405
>  worker_thread+0x8e0/0xfe8 kernel/workqueue.c:2552
>  kthread+0x288/0x310 kernel/kthread.c:379
>  ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:853
> 
> This fix let bit_cursor() bail out early when a font bitmap
> isn't available yet.
> 
> Signed-off-by: Helge Deller <deller at gmx.de>
> Cc: stable at kernel.org
> Reported-by: syzbot+d910bd780e6efac35869 at syzkaller.appspotmail.com
> ---
>  drivers/video/fbdev/core/bitblit.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
> index f98e8f298bc1..8587c9da0670 100644
> --- a/drivers/video/fbdev/core/bitblit.c
> +++ b/drivers/video/fbdev/core/bitblit.c
> @@ -247,6 +247,9 @@ static void bit_cursor(struct vc_data *vc, struct fb_info *info, int mode,
> 
>  	cursor.set = 0;
> 
> +	if (!vc->vc_font.data)
> +		return;
> +
>   	c = scr_readw((u16 *) vc->vc_pos);
>  	attribute = get_attribute(info, c);
>  	src = vc->vc_font.data + ((c & charmask) * (w * vc->vc_font.height));

When browsing the code I noticed another use of vc->vc_font.data.

bit_putcs_{un}aligned() access vc->vc_font.data too, so I wonder if
bit_putcs() should also check for vc->vc_font.data?
bit_putcs() is the only user of bit_putcs_{un}aligned.


The actual patch here looks good from a quick browsing around, but I
cannot claim I did a review. But you have my:
Acked-by: Sam Ravnborg <sam at ravnborg.org>

	Sam