[PATCH v3] tty: Fix out-of-bound vmalloc access in imageblit

[Igor Matheus Andrade Torrente <igormtorrente@gmail.com>]

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 made the resize_screen() be called
even if the screen does not need any resizing, so it will
"fix and fill" the fb_var_screeninfo independently.

Reported-and-tested-by: syzbot+858dc7a2f7ef07c2c219@syzkaller.appspotmail.com
Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
---
v2: It Tries to avoid the problem found by Greg in the previous
    patch.

v3: Add a big comment explaining why this is being done the way
    it is.
---
 drivers/tty/vt/vt.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index fa1548d4f94b..a0207525a945 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1220,7 +1220,20 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
 	new_screen_size = new_row_size * new_rows;
 
 	if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
-		return 0;
+		/* This function is being called here to cover the case
+		 * where the userspace calls the FBIOPUT_VSCREENINFO twice,
+		 * passing the same fb_var_screeninfo containing only the
+		 * fields xres, yres, and bits_per_pixel with yeird(but valid)
+		 * values.
+		 * In the second call, the struct fb_var_screeninfo isn't
+		 * being modified by the underlying driver because of the
+		 * if above, and this eventually causes out-of-bound
+		 * access by the imageblit function.
+		 * To give the correct values to the struct and to not have
+		 * to deal with possible erros from the code bellow, we call
+		 * the resize_screen here as well.
+		 */
+		return resize_screen(vc, new_cols, new_rows, user);
 
 	if (new_screen_size > KMALLOC_MAX_SIZE || !new_screen_size)
 		return -EINVAL;
-- 
2.20.1