* [PATCH v5] tty: Fix out-of-bound vmalloc access in imageblit
@ 2021-06-28 13:45 Igor Matheus Andrade Torrente
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From: Igor Matheus Andrade Torrente @ 2021-06-28 13:45 UTC (permalink / raw)
To: gregkh, jirislaby
Cc: Igor Matheus Andrade Torrente, linux-kernel, syzbot+858dc7a2f7ef07c2c219
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>
---
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 fa1548d4f94b..f384d251967f 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;
--
2.20.1
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