Re: [RFC PATCH] fbcon: Fix out-of-bounds memory in fbcon_putcs

[chenjun (AM)]

在 2026/3/2 19:34, Thomas Zimmermann 写道:
> Hi
> 
> Am 02.03.26 um 12:24 schrieb chenjun (AM):
>> 在 2026/3/2 18:19, Thomas Zimmermann 写道:
>>>
>>> Am 27.02.26 um 15:43 schrieb Chen Jun:
>>>> When a font is set on an invisible console, the screen will not update.
>>>> However, the fontbuffer is not updated to match the new font dimensions.
>>>>
>>>> This inconsistency leads to out-of-bounds memory access when writing to
>>>> the tty bound to fbcon, as demonstrated by the following KASAN report:
>>>>
>>>> BUG: KASAN: slab-out-of-bounds in fb_pad_aligned_buffer+0xdf/0x140
>>>> Read of size 1 at addr ffff8881195a2280 by task a.out/971
>>>> Call Trace:
>>>>      <TASK>
>>>>      fb_pad_aligned_buffer+0xdf/0x140
>>>>      ud_putcs+0x88a/0xde0
>>>>      fbcon_putcs+0x319/0x430
>>>>      do_update_region+0x23c/0x3b0
>>>>      do_con_write+0x225c/0x67f0
>>>>      con_write+0xe/0x30
>>>>      n_tty_write+0x4b5/0xff0
>>>>      file_tty_write.isra.41+0x46c/0x880
>>>>      vfs_write+0x868/0xd60
>>>>      ksys_write+0xf2/0x1d0
>>>>      do_syscall_64+0xfa/0x570
>>>>
>>>> Fix this by calling fbcon_rotate_font() if vc is invisible in
>>>> fbcon_do_set_font().
>>>>
>>>> Signed-off-by: Chen Jun <chenjun102@xxxxxxxxxx>
>>> Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
>> Hi Thomas,
>>
>> Thanks for your review.
>>
>> I'm not familiar with the fbcon module. Is there a better way to fix this?
> 
> Not really, I think. The whole module first needs a redesign to be
> easier to understand.
> 
> Best regards
> Thomas
> 
>>
>>>> ---
>>>>      drivers/video/fbdev/core/fbcon.c | 5 +++++
>>>>      1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
>>>> index 666261ae59d8..d76100188bee 100644
>>>> --- a/drivers/video/fbdev/core/fbcon.c
>>>> +++ b/drivers/video/fbdev/core/fbcon.c
>>>> @@ -2444,6 +2444,11 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
>>>>      		rows = FBCON_SWAP(par->rotate, info->var.yres, info->var.xres);
>>>>      		cols /= w;
>>>>      		rows /= h;
>>>> +		if (!con_is_visible(vc)) {
>>>> +			ret = fbcon_rotate_font(info, vc);
>>>> +			if (ret)
>>>> +				goto err_out;
>>>> +		}

Hi Thomas and Helge,

I apologize, but after reviewing the code, I believe there is a problem.
The issue is that fbcon_do_set_font() updates members of 
info->fbcon_par, and the info are shared with other vc instances.

Best regards
Chen Jun

>>>>      		ret = vc_resize(vc, cols, rows);
>>>>      		if (ret)
>>>>      			goto err_out;
>>
>