video: fbdev: s3fb: Check the size of screen before memset_io()

In the function s3fb_set_par(), the value of 'screen_size' is
calculated by the user input. If the user provides the improper value,
the value of 'screen_size' may larger than 'info->screen_size', which
may cause the following bug:

[   54.083733] BUG: unable to handle page fault for address: ffffc90003000000
[   54.083742] #PF: supervisor write access in kernel mode
[   54.083744] #PF: error_code(0x0002) - not-present page
[   54.083760] RIP: 0010:memset_orig+0x33/0xb0
[   54.083782] Call Trace:
[   54.083788]  s3fb_set_par+0x1ec6/0x4040
[   54.083806]  fb_set_var+0x604/0xeb0
[   54.083836]  do_fb_ioctl+0x234/0x670

Fix the this by checking the value of 'screen_size' before memset_io().

Fixes: a268422de8 ("fbdev driver for S3 Trio/Virge")
Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/video/fbdev/s3fb.c

@@ -905,6 +905,8 @@ static int s3fb_set_par(struct fb_info *info)
 	value = clamp((htotal + hsstart + 1) / 2 + 2, hsstart + 4, htotal + 1);
 	svga_wcrt_multi(par->state.vgabase, s3_dtpc_regs, value);
 
+	if (screen_size > info->screen_size)
+		screen_size = info->screen_size;
 	memset_io(info->screen_base, 0x00, screen_size);
 	/* Device and screen back on */
 	svga_wcrt_mask(par->state.vgabase, 0x17, 0x80, 0x80);