[PATCH RFC] tty: Fix out-of-bound vmalloc access in imageblit

[Igor Matheus Andrade Torrente]

This issue happens when a userspace program does an ioctl
FBIOPUT_VSCREENINFO passing the fb_var_screeninfo struct
containing only the fields xres, yres, and bits_per_pixel
with values.

If this struct is the same as the previous ioctl, the
vc_resize() detects it and doesn't call the resize_screen(),
leaving the fb_var_screeninfo incomplete. And this leads to
the updatescrollmode() calculates a wrong value to
fbcon_display->vrows, which makes the real_y() return a
wrong value of y, and that value, eventually, causes
the imageblit to access an out-of-bound address value.

To solve this issue I brougth the resize_screen() the
beginning of vc_do_resize(), so it will "fix and fill"
the fb_var_screeninfo even if the screen does not need any
resizing.

Reported-and-tested-by: syzbot+858dc7a2f7ef07c2c219@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@xxxxxxxxx>
---
 drivers/tty/vt/vt.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index fa1548d4f94b..1b90758d8893 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1219,6 +1219,10 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
 	new_row_size = new_cols << 1;
 	new_screen_size = new_row_size * new_rows;
 
+	err = resize_screen(vc, new_cols, new_rows, user);
+	if (err)
+		return err;
+
 	if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
 		return 0;
 
@@ -1241,14 +1245,6 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
 
 	old_rows = vc->vc_rows;
 	old_row_size = vc->vc_size_row;
-
-	err = resize_screen(vc, new_cols, new_rows, user);
-	if (err) {
-		kfree(newscreen);
-		vc_uniscr_free(new_uniscr);
-		return err;
-	}
-
 	vc->vc_rows = new_rows;
 	vc->vc_cols = new_cols;
 	vc->vc_size_row = new_row_size;
-- 
2.20.1