[PATCH 5.14 027/172] tty: Fix out-of-bound vmalloc access in imageblit

Greg Kroah-Hartman
2021-10-04T13:32:26Z

From: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>

[ Upstream commit 3b0c406124719b625b1aba431659f5cdc24a982c ]

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.

Cc: stable <stable@vger.kernel.org> # after 5.15-rc2 is out, give it time to bake
Reported-and-tested-by: syzbot+858dc7a2f7ef07c2c219@syzkaller.appspotmail.com
Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
Link: https://lore.kernel.org/r/20210628134509.15895-1-igormtorrente@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/vt/vt.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index cb72393f92d3..153d4a88ec9a 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1219,8 +1219,25 @@ 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;

- if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
- return 0;
+ if (new_cols == vc->vc_cols && new_rows == vc->vc_rows) {
+ /*
+ * 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 the fields
+ * yres/xres equal to a number non-multiple of vc_font.height
+ * and yres_virtual/xres_virtual equal to number lesser than the
+ * vc_font.height and yres/xres.
+ * In the second call, the struct fb_var_screeninfo isn't
+ * being modified by the underlying driver because of the
+ * if above, and this causes the fbcon_display->vrows to become
+ * negative and it eventually leads to out-of-bound
+ * access by the imageblit function.
+ * To give the correct values to the struct and to not have
+ * to deal with possible errors from the code below, 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;
--
2.33.0