[PATCH] vgacon: fix a UAF in do_update_region()

[Yang Yingliang]

I got a UAF report in do_update_region() when I doing fuzz test.

[   51.161905] BUG: KASAN: use-after-free in do_update_region+0x579/0x600
[   51.161918] Read of size 2 at addr ffff888000100000 by task test/295

[   51.161957] CPU: 2 PID: 295 Comm: test Not tainted 5.7.0+ #975
[   51.161969] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
[   51.161976] Call Trace:
[   51.162001]  dump_stack+0xc6/0x11e
[   51.162019]  ? do_update_region+0x579/0x600
[   51.162047]  print_address_description.constprop.6+0x1a/0x220
[   51.162083]  ? vprintk_func+0x66/0xed
[   51.162100]  ? do_update_region+0x579/0x600
[   51.162112]  ? do_update_region+0x579/0x600
[   51.162128]  kasan_report.cold.9+0x37/0x7c
[   51.162151]  ? do_update_region+0x579/0x600
[   51.162173]  do_update_region+0x579/0x600
[   51.162207]  ? con_get_trans_old+0x230/0x230
[   51.162229]  ? retint_kernel+0x10/0x10
[   51.162278]  csi_J+0x557/0xa00
[   51.162307]  do_con_trol+0x49af/0x5cc0
[   51.162330]  ? lock_downgrade+0x720/0x720
[   51.162347]  ? reset_palette+0x1b0/0x1b0
[   51.162369]  ? lockdep_hardirqs_on_prepare+0x379/0x540
[   51.162393]  ? notifier_call_chain+0x11b/0x160
[   51.162438]  do_con_write.part.24+0xb0a/0x1a30
[   51.162501]  ? do_con_trol+0x5cc0/0x5cc0
[   51.162522]  ? console_unlock+0x7b8/0xb00
[   51.162555]  ? __mutex_unlock_slowpath+0xd4/0x670
[   51.162574]  ? this_tty+0xe0/0xe0
[   51.162589]  ? console_unlock+0x559/0xb00
[   51.162605]  ? wait_for_completion+0x260/0x260
[   51.162638]  con_write+0x31/0xb0
[   51.162658]  n_tty_write+0x4fa/0xd40
[   51.162710]  ? n_tty_read+0x1800/0x1800
[   51.162730]  ? prepare_to_wait_exclusive+0x270/0x270
[   51.162754]  ? __might_fault+0x175/0x1b0
[   51.162783]  tty_write+0x42b/0x8d0
[   51.162795]  ? n_tty_read+0x1800/0x1800
[   51.162825]  ? tty_lookup_driver+0x450/0x450
[   51.162848]  __vfs_write+0x7c/0x100
[   51.162875]  vfs_write+0x1c9/0x510
[   51.162901]  ksys_write+0xff/0x200
[   51.162918]  ? __ia32_sys_read+0xb0/0xb0
[   51.162940]  ? do_syscall_64+0x1a/0x520
[   51.162957]  ? lockdep_hardirqs_on_prepare+0x379/0x540
[   51.162984]  do_syscall_64+0xa1/0x520
[   51.163008]  entry_SYSCALL_64_after_hwframe+0x49/0xb3

After vgacon_set_origin() is called in set_origin(), the vc_origin is
set to vga_vram_base, the vc_pos should between vga_vram_base and
vga_vram_end. But we still use vc_screenbuf_size, if the vga_vram_size
is smaller than vc_screenbuf_size, vc_pos may be out of bound, using it
will cause a use-after-free(or out-of-bounds). Fix this by calling
vc_resize() if vga_vram_size is smaller than vc_screenbuf_size.

Signed-off-by: Yang Yingliang <yangyingliang@xxxxxxxxxx>
---
 drivers/video/console/vgacon.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index 998b0de..2ee3d62 100644
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c
@@ -1336,6 +1336,9 @@ static int vgacon_set_origin(struct vc_data *c)
 	if (vga_is_gfx ||	/* We don't play origin tricks in graphic modes */
 	    (console_blanked && !vga_palette_blanked))	/* Nor we write to blanked screens */
 		return 0;
+
+	if (c->vc_screenbuf_size > vga_vram_size)
+		vc_resize(c, screen_info.orig_video_cols, screen_info.orig_video_lines);
 	c->vc_origin = c->vc_visible_origin = vga_vram_base;
 	vga_set_mem_top(c);
 	vga_rolled_over = 0;
-- 
1.8.3